Use a buildcustomize.pl to set @INC in miniperl when building extensions.

With the build tools now shipped in various subdirectories of cpan/ and dist/
we need to add several paths to @INC when invoking MakeMaker (etc) to build
extensions.

The previous approach of using $ENV{PERL5LIB} was fragile, because:
a: It was hitting the length limit for %ENV variables on VMS
b: It was running the risk of race conditions in a parallel build -
   ExtUtils::Makemaker "knows" to add -I../..lib, which puts lib at the *front*
   of @INC, but if one parallel process happens to copy a module into lib/
   whilst another is searching for it, the second may get a partial read
c: Overwriting $ENV{PERL5LIB} breaks any system where any of the installed
   build tools are actually implemented in Perl, if they are relying on
   $ENV{PERL5LIB} for setup

This approach

a: Doesn't have %ENV length limits
b: Ensures that lib/ is last, so copy targets are always shadowing copy
   sources
c: Only affects miniperl, and doesn't touch $ENV{PERL5LIB}

Approaches that turned out to have fatal flaws:

1: Using $ENV{PERL5OPT} with a module fails because ExtUtils::MakeMaker
   searches for the build perl without setting lib, and treats the error
   caused by a failed -M as "not a valid perl 5 binary"
2: Refactoring ExtUtils::MakeMaker to *not* use -I for lib, and instead rely
   on $ENV{PERL5LIB} [which includes "../../lib"] fails because:
   some extensions have subdirectories, and on these EU::MM correctly uses
   -I../../../lib, where as $ENV{PERL5LIB} only has space for relative paths,
   and only with two levels.

This approach actually takes advantage of ExtUtils::MakeMaker setting an -I
option correct for the depth of directory being built.

9 files changed, 97 insertions, 57 deletions

diff --git a/Cross/Makefile-cross-SH b/Cross/Makefile-cross-SH
index 2ea58f5a9d..d1d2cbf7a1 100644
--- a/Cross/Makefile-cross-SH
+++ b/Cross/Makefile-cross-SH
@@ -324,7 +324,7 @@ $spitshell >>$Makefile <<'!NO!SUBS!'
 
 CONFIGPM = xlib/$(CROSS_NAME)/Config.pm
 
-private = preplibrary $(CONFIGPM) $(CROSS_LIB)/Config.pod
+private = preplibrary $(CONFIGPM) $(CROSS_LIB)/Config.pod lib/buildcustomize.pl
 
 shextract = Makefile cflags config.h makedepend \
 	makedir myconfig writemain pod/Makefile
@@ -738,6 +738,9 @@ lib/lib.pm:	miniperl $(CONFIGPM)
 	@-rm -f $@
 	$(LDLIBPTH) ./miniperl -Ilib -MCross lib/lib_pm.PL
 
+lib/buildcustomize.pl: $(MINIPERL_EXE) write_buildcustomize.pl
+	$(MINIPERL) write_buildcustomize.pl >lib/buildcustomize.pl
+
 unidatafiles $(unidatafiles): uni.data
 
 uni.data: miniperl$(EXE_EXT) $(CONFIGPM) lib/unicore/mktables
@@ -904,16 +907,16 @@ manicheck:	FORCE
 
 
 
-$(DYNALOADER):	preplibrary FORCE
+$(DYNALOADER): lib/buildcustomize.pl preplibrary FORCE
 	@$(LDLIBPTH) $(RUN) ./miniperl$(EXE_EXT) -Ilib make_ext.pl --cross $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL) LINKTYPE=static $(STATIC_LDFLAGS)
 
-d_dummy $(dynamic_ext):	miniperl$(EXE_EXT) preplibrary makeppport $(DYNALOADER) FORCE
+d_dummy $(dynamic_ext):	miniperl$(EXE_EXT) lib/buildcustomize.pl preplibrary makeppport $(DYNALOADER) FORCE
 	@$(LDLIBPTH) $(RUN) ./miniperl$(EXE_EXT) -Ilib make_ext.pl --cross $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL) LINKTYPE=dynamic
 
-s_dummy $(static_ext):	miniperl$(EXE_EXT) preplibrary makeppport $(DYNALOADER) FORCE
+s_dummy $(static_ext):	miniperl$(EXE_EXT) lib/buildcustomize.pl preplibrary makeppport $(DYNALOADER) FORCE
 	@$(LDLIBPTH) $(RUN) ./miniperl$(EXE_EXT) -Ilib make_ext.pl --cross $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL) LINKTYPE=static $(STATIC_LDFLAGS)
 
-n_dummy $(nonxs_ext):	miniperl$(EXE_EXT) preplibrary $(DYNALOADER) FORCE
+n_dummy $(nonxs_ext):	miniperl$(EXE_EXT) lib/buildcustomize.pl preplibrary $(DYNALOADER) FORCE
 	@$(LDLIBPTH) $(RUN) ./miniperl$(EXE_EXT) -Ilib make_ext.pl --cross $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL)
 !NO!SUBS!
 
diff --git a/MANIFEST b/MANIFEST
index 982fab7ac8..11323a752e 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -5180,6 +5180,7 @@ win32/win32thread.h		Win32 port mapping to threads
 win32/wince.c			WinCE port
 win32/wince.h			WinCE port
 win32/wincesck.c		WinCE port
+write_buildcustomize.pl		Generate lib/buildcustomize.pl
 x2p/a2p.c			Output of a2p.y run through byacc
 x2p/a2p.h			Global declarations
 x2p/a2p.pod			Pod for awk to perl translator
diff --git a/Makefile.SH b/Makefile.SH
index c8ad2a8f52..6577cc3e58 100755
--- a/Makefile.SH
+++ b/Makefile.SH
@@ -419,7 +419,7 @@ esac
 ## In the following dollars and backticks do not need the extra backslash.
 $spitshell >>$Makefile <<'!NO!SUBS!'
 
-private = preplibrary $(CONFIGPM) $(CONFIGPOD) lib/ExtUtils/Miniperl.pm git_version.h
+private = preplibrary $(CONFIGPM) $(CONFIGPOD) lib/ExtUtils/Miniperl.pm git_version.h lib/buildcustomize.pl
 
 # Files to be built with variable substitution before miniperl
 # is available.
@@ -999,6 +999,9 @@ lib/re.pm: ext/re/re.pm
 	@-rm -f $@
 	cp ext/re/re.pm lib/re.pm
 
+lib/buildcustomize.pl: $(MINIPERL_EXE) write_buildcustomize.pl
+	$(MINIPERL) write_buildcustomize.pl >lib/buildcustomize.pl
+
 unidatafiles $(unidatafiles) pod/perluniprops.pod: uni.data
 
 uni.data: $(MINIPERL_EXE) $(CONFIGPM) lib/unicore/mktables $(nonxs_ext)
@@ -1167,16 +1170,16 @@ manicheck:	FORCE
 #
 # DynaLoader may be needed for extensions that use Makefile.PL.
 
-$(DYNALOADER):	$(MINIPERL_EXE) preplibrary FORCE $(nonxs_ext)
+$(DYNALOADER):	$(MINIPERL_EXE) lib/buildcustomize.pl preplibrary FORCE $(nonxs_ext)
 	$(MINIPERL) make_ext.pl $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL) LINKTYPE=static $(STATIC_LDFLAGS)
 
-d_dummy $(dynamic_ext):	$(MINIPERL_EXE) preplibrary makeppport $(DYNALOADER) FORCE $(PERLEXPORT)
+d_dummy $(dynamic_ext):	$(MINIPERL_EXE) lib/buildcustomize.pl preplibrary makeppport $(DYNALOADER) FORCE $(PERLEXPORT)
 	$(MINIPERL) make_ext.pl $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL) LINKTYPE=dynamic
 
-s_dummy $(static_ext):	$(MINIPERL_EXE) preplibrary makeppport $(DYNALOADER) FORCE
+s_dummy $(static_ext):	$(MINIPERL_EXE) lib/buildcustomize.pl preplibrary makeppport $(DYNALOADER) FORCE
 	$(MINIPERL) make_ext.pl $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL) LINKTYPE=static $(STATIC_LDFLAGS)
 
-n_dummy $(nonxs_ext):	$(MINIPERL_EXE) preplibrary FORCE
+n_dummy $(nonxs_ext):	$(MINIPERL_EXE) lib/buildcustomize.pl preplibrary FORCE
 	$(MINIPERL) make_ext.pl $@ MAKE=$(MAKE) LIBPERL_A=$(LIBPERL)
 !NO!SUBS!
 
diff --git a/installperl b/installperl
index 74856aab18..f29f8e512f 100755
--- a/installperl
+++ b/installperl
@@ -714,6 +714,8 @@ sub installlib {
     # ignore the test extensions
     return if $dir =~ m{\bXS/(?:APItest|Typemap)\b};
     return if $name =~ m{\b(?:APItest|Typemap)\.pm$};
+    # ignore the build support code
+    return if $name =~ /\bbuildcustomize\.pl$/;
     # ignore the demo files
     return if $dir =~ /\b(?:demos?|eg)\b/;
     # ignore unneeded unicore files
diff --git a/make_ext.pl b/make_ext.pl
index b780ec9dab..13a15b4601 100644
--- a/make_ext.pl
+++ b/make_ext.pl
@@ -4,11 +4,9 @@ use warnings;
 use Config;
 BEGIN {
     if ($^O eq 'MSWin32') {
-	unshift @INC, ('../dist/Cwd', '../dist/Cwd/lib');
-	require File::Spec::Functions;
+	unshift @INC, '../dist/Cwd';
 	require FindExt;
-    }
-    else {
+    } else {
 	unshift @INC, 'dist/Cwd';
     }
 }
@@ -18,27 +16,6 @@ my $is_Win32 = $^O eq 'MSWin32';
 my $is_VMS = $^O eq 'VMS';
 my $is_Unix = !$is_Win32 && !$is_VMS;
 
-# To clarify, this isn't the entire suite of modules considered "toolchain"
-# It's not even all modules needed to build ext/
-# It's just the source paths of the (minimum complete set of) modules in ext/
-# needed to build the nonxs modules
-# After which, all nonxs modules are in lib, which was always sufficient to
-# allow miniperl to build everything else.
-
-# This list cannot get any longer without overflowing the length limit for
-# environment variables on VMS
-my @toolchain = qw(cpan/AutoLoader/lib
-		   dist/Cwd dist/Cwd/lib
-		   dist/ExtUtils-Command/lib
-		   dist/ExtUtils-Install/lib
-		   cpan/ExtUtils-MakeMaker/lib
-		   dist/ExtUtils-Manifest/lib
-		   cpan/File-Path/lib
-		   );
-
-# Used only in ExtUtils::Liblist::Kid::_win32_ext()
-push @toolchain, 'cpan/Text-ParseWords/lib' if $is_Win32;
-
 my @ext_dirs = qw(cpan dist ext);
 my $ext_dirs_re = '(?:' . join('|', @ext_dirs) . ')';
 
@@ -296,16 +273,7 @@ sub build_extension {
     $perl ||= "$up/miniperl";
     my $return_dir = $up;
     my $lib_dir = "$up/lib";
-    # $lib_dir must be last, as we're copying files into it, and in a parallel
-    # make there's a race condition if one process tries to open a module that
-    # another process has half-written.
-    my @new_inc = ((map {"$up/$_"} @toolchain), $lib_dir);
-    if ($is_Win32) {
-	@new_inc = map {File::Spec::Functions::rel2abs($_)} @new_inc;
-    }
-    $ENV{PERL5LIB} = join $Config{path_sep}, @new_inc;
     $ENV{PERL_CORE} = 1;
-    # warn $ENV{PERL5LIB};
 
     my $makefile;
     if ($is_VMS) {
@@ -421,7 +389,7 @@ EOM
 	    @cross = '-MCross';
 	}
 	    
-	my @args = (@cross, 'Makefile.PL');
+	my @args = ("-I$lib_dir", @cross, 'Makefile.PL');
 	if ($is_VMS) {
 	    my $libd = VMS::Filespec::vmspath($lib_dir);
 	    push @args, "INST_LIB=$libd", "INST_ARCHLIB=$libd";
diff --git a/vms/descrip_mms.template b/vms/descrip_mms.template
index 82fddde428..846afa49c8 100644
--- a/vms/descrip_mms.template
+++ b/vms/descrip_mms.template
@@ -361,7 +361,7 @@ CRTLOPTS =,$(CRTL)/Options
 unidatadirs = lib/unicore/To lib/unicore/lib
 
 # Modules which must be installed before we can build extensions
-LIBPREREQ = $(ARCHDIR)Config.pm $(ARCHDIR)Config_heavy.pl [.lib.VMS]Filespec.pm $(ARCHDIR)vmspipe.com [.lib]re.pm
+LIBPREREQ = $(ARCHDIR)Config.pm $(ARCHDIR)Config_heavy.pl [.lib.VMS]Filespec.pm $(ARCHDIR)vmspipe.com [.lib]re.pm [.lib]buildcustomize.pl
 
 utils1 = [.lib.pods]perldoc.com [.lib.ExtUtils]Miniperl.pm [.utils]c2ph.com [.utils]h2ph.com 
 utils2 = [.utils]h2xs.com [.utils]libnetcfg.com [.lib]perlbug.com [.utils]dprofpp.com  [.utils]json_pp.com
@@ -460,6 +460,9 @@ archcorefiles : $(ac) $(ARCHAUTO)time.stamp
 [.lib]re.pm  : [.ext.re]re.pm
 	Copy/NoConfirm/Log $(MMS$SOURCE) [.lib]
 
+[.lib]buildcustomize.pl : write_buildcustomize.pl $(MINIPERL_EXE)
+       $(MINIPERL) write_buildcustomize.pl > [.lib]buildcustomize.pl
+
 vmspipe.com : [.vms]vmspipe.com
 	Copy/NoConfirm/Log $(MMS$SOURCE) []
 
@@ -566,7 +569,7 @@ unidatafiles.ts : $(MINIPERL_EXE) [.lib]Config.pm [.lib.unicore]mktables nonxsex
 	@ If F$Search("$(MMS$TARGET)").nes."" Then Delete/NoLog/NoConfirm $(MMS$TARGET);*
 	@ Copy/NoConfirm _NLA0: $(MMS$TARGET)
  
-DynaLoader$(O) : $(ARCHDIR)Config.pm $(MINIPERL_EXE) [.lib.VMS]Filespec.pm 
+DynaLoader$(O) : [.lib]buildcustomize.pl $(ARCHDIR)Config.pm $(MINIPERL_EXE) [.lib.VMS]Filespec.pm 
 	$(MINIPERL) make_ext.pl "MAKE=$(MMS)" "DynaLoader"
 
 dynext : $(LIBPREREQ) $(DBG)perlshr$(E) unidatafiles.ts DynaLoader$(O) preplibrary makeppport $(MINIPERL_EXE)
@@ -1896,6 +1899,7 @@ tidy : cleanlis
 	- If F$Search("[.utils]*.com;-1").nes."" Then Purge/NoConfirm/Log [.utils]*.com
 	- If F$Search("[.x2p]*.com;-1").nes."" Then Purge/NoConfirm/Log [.x2p]*.com
 	- If F$Search("[.lib.pods]*.com;-1").nes."" Then Purge/NoConfirm/Log [.lib.pods]*.com
+	- If F$Search("[.lib]buildcustomize.pl;-1").nes."" Then Purge/NoConfirm/Log [.lib]buildcustomize.pl
 
 clean : tidy cleantest
       	- $(MINIPERL) make_ext.pl "MAKE=$(MMS)" "--all" "--target=clean"
diff --git a/win32/Makefile b/win32/Makefile
index ceaaaa5ca4..4897a4ad35 100644
--- a/win32/Makefile
+++ b/win32/Makefile
@@ -858,6 +858,9 @@ $(CONFIGPM) : $(MINIPERL) ..\config.sh config_h.PL ..\minimod.pl
 	-$(MINIPERL) -I..\lib $(ICWD) config_h.PL "INST_VER=$(INST_VER)"
 	if errorlevel 1 $(MAKE) /$(MAKEFLAGS) $(CONFIGPM)
 
+..\lib\buildcustomize.pl: $(MINIPERL) ..\write_buildcustomize.pl
+	$(MINIPERL) -I..\lib ..\write_buildcustomize.pl .. >..\lib\buildcustomize.pl
+
 $(MINIPERL) : $(MINIDIR) $(MINI_OBJ)
 	$(LINK32) -subsystem:console -out:$@ @<<
 	$(LINK_FLAGS) $(LIBFILES) $(MINI_OBJ)
@@ -982,24 +985,24 @@ MakePPPort: $(MINIPERL) $(CONFIGPM) Extensions_nonxs
 #-------------------------------------------------------------------------------
 # There's no direct way to mark a dependency on
 # DynaLoader.pm, so this will have to do
-Extensions: ..\make_ext.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
+Extensions: ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --dynamic
 
-Extensions_reonly: ..\make_ext.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
+Extensions_reonly: ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --dynamic +re
 
-Extensions_static : ..\make_ext.pl list_static_libs.pl $(PERLDEP) $(CONFIGPM)
+Extensions_static : ..\make_ext.pl ..\lib\buildcustomize.pl list_static_libs.pl $(PERLDEP) $(CONFIGPM)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --static
 	$(MINIPERL) -I..\lib list_static_libs.pl > Extensions_static
 
-Extensions_nonxs: ..\make_ext.pl $(PERLDEP) $(CONFIGPM)
+Extensions_nonxs: ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --nonxs
 
-$(DYNALOADER) : ..\make_ext.pl $(PERLDEP) $(CONFIGPM) Extensions_nonxs
+$(DYNALOADER) : ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM) Extensions_nonxs
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(EXTDIR) --dynaloader
 
@@ -1094,6 +1097,7 @@ distclean: realclean
 	-del /f $(LIBDIR)\Win32CORE.pm
 	-del /f $(LIBDIR)\Win32API\File.pm
 	-del /f $(LIBDIR)\Win32API\File\cFile.pc
+	-del /f $(LIBDIR)\buildcustomize.pl
 	-del /f $(DISTDIR)\XSLoader\XSLoader.pm
 	-if exist $(LIBDIR)\App rmdir /s /q $(LIBDIR)\App
 	-if exist $(LIBDIR)\Archive rmdir /s /q $(LIBDIR)\Archive
diff --git a/win32/makefile.mk b/win32/makefile.mk
index f3fface05a..fcc30ac473 100644
--- a/win32/makefile.mk
+++ b/win32/makefile.mk
@@ -1160,6 +1160,10 @@ $(CONFIGPM) : $(MINIPERL) ..\config.sh config_h.PL ..\minimod.pl
 	$(MINIPERL) -I..\lib $(ICWD) config_h.PL "INST_VER=$(INST_VER)" \
 	    || $(MAKE) $(MAKEMACROS) $(CONFIGPM) $(MAKEFILE)
 
+..\lib\buildcustomize.pl: $(MINIPERL) ..\write_buildcustomize.pl
+	$(MINIPERL) -I..\lib ..\write_buildcustomize.pl .. >..\lib\buildcustomize.pl
+
+
 $(MINIPERL) : $(MINIDIR) $(MINI_OBJ) $(CRTIPMLIBS)
 .IF "$(CCTYPE)" == "BORLAND"
 	if not exist $(CCLIBDIR)\PSDK\odbccp32.lib \
@@ -1370,24 +1374,24 @@ MakePPPort: $(MINIPERL) $(CONFIGPM) Extensions_nonxs
 #-------------------------------------------------------------------------------
 # There's no direct way to mark a dependency on
 # DynaLoader.pm, so this will have to do
-Extensions : ..\make_ext.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
+Extensions : ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --dynamic
 
-Extensions_reonly : ..\make_ext.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
+Extensions_reonly : ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM) $(DYNALOADER)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --dynamic +re
 
-Extensions_static : ..\make_ext.pl list_static_libs.pl $(PERLDEP) $(CONFIGPM)
+Extensions_static : ..\make_ext.pl ..\lib\buildcustomize.pl list_static_libs.pl $(PERLDEP) $(CONFIGPM)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --static
 	$(MINIPERL) -I..\lib list_static_libs.pl > Extensions_static
 
-Extensions_nonxs : ..\make_ext.pl $(PERLDEP) $(CONFIGPM)
+Extensions_nonxs : ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM)
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(CPANDIR) --dir=$(DISTDIR) --dir=$(EXTDIR) --nonxs
 
-$(DYNALOADER) : ..\make_ext.pl $(PERLDEP) $(CONFIGPM) Extensions_nonxs
+$(DYNALOADER) : ..\make_ext.pl ..\lib\buildcustomize.pl $(PERLDEP) $(CONFIGPM) Extensions_nonxs
 	$(XCOPY) ..\*.h $(COREDIR)\*.*
 	$(MINIPERL) -I..\lib ..\make_ext.pl "MAKE=$(MAKE)" --dir=$(EXTDIR) --dynaloader
 
@@ -1479,6 +1483,7 @@ distclean: realclean
 	-del /f $(LIBDIR)\Win32CORE.pm
 	-del /f $(LIBDIR)\Win32API\File.pm
 	-del /f $(LIBDIR)\Win32API\File\cFile.pc
+	-del /f $(LIBDIR)\buildcustomize.pl
 	-del /f $(DISTDIR)\XSLoader\XSLoader.pm
 	-if exist $(LIBDIR)\App rmdir /s /q $(LIBDIR)\App
 	-if exist $(LIBDIR)\Archive rmdir /s /q $(LIBDIR)\Archive
diff --git a/write_buildcustomize.pl b/write_buildcustomize.pl
new file mode 100644
index 0000000000..7d7589645d
--- /dev/null
+++ b/write_buildcustomize.pl
@@ -0,0 +1,50 @@
+#!./miniperl -w
+
+use strict;
+if (@ARGV) {
+    my $dir = shift;
+    chdir $dir or die "Can't chdir '$dir': $!";
+    unshift @INC, 'lib';
+}
+
+unshift @INC, ('dist/Cwd', 'dist/Cwd/lib');
+require File::Spec::Functions;
+
+# To clarify, this isn't the entire suite of modules considered "toolchain"
+# It's not even all modules needed to build ext/
+# It's just the source paths of the (minimum complete set of) modules in ext/
+# needed to build the nonxs modules
+# After which, all nonxs modules are in lib, which was always sufficient to
+# allow miniperl to build everything else.
+
+my @toolchain = qw(cpan/AutoLoader/lib
+		   dist/Cwd dist/Cwd/lib
+		   dist/ExtUtils-Command/lib
+		   dist/ExtUtils-Install/lib
+		   cpan/ExtUtils-MakeMaker/lib
+		   dist/ExtUtils-Manifest/lib
+		   cpan/File-Path/lib
+		   );
+
+# Used only in ExtUtils::Liblist::Kid::_win32_ext()
+push @toolchain, 'cpan/Text-ParseWords/lib' if $^O eq 'MSWin32';
+
+# lib must be last, as the the toolchain modules write themselves into it
+# as they build, and it's important that @INC order ensures that the partially
+# written files are always masked by the complete versions.
+
+my $inc = join ",\n        ",
+    map { "q\0$_\0" }
+    (map {File::Spec::Functions::rel2abs($_)} @toolchain, 'lib'), '.';
+
+# If any of the system's build tools are written in Perl, then this module
+# may well be loaded by a much older version than we are building. So keep it
+# as backwards compatible as is easy.
+print <<"EOT";
+#!perl
+
+# We are miniperl, building extensions
+# Reset \@INC completely, adding the directories we need, and removing the
+# installed directories (which we don't need to read, and may confuse us)
+\@INC = ($inc);
+EOT