Fix regression that broke Automake ‘make check’

Problem reported by Ken Moffat (sr#110287); the problem was
introduced in 2016-12-21T16:15:46Z!daniel.kitta@gmail.com.
* bin/autoheader.in (templates_for_header):
When generating warnings about symbols lacking templates,
downgrade template read failure from a fatal error to a warning.
Also, don’t even try to read from a template file whose name has
shell metavariables (which Autoconf 2.50 withdrew support for);
just warn about that, too.  These changes cause the Automake
tests to merely generate warnings that are ignored, instead
of failing.
* doc/autoconf.texi (Configuration Files, Configuration Headers)
(Configuration Commands, Configuration Links):
Also document here that the file names should not contain
shell metacharacters, to make this constraint more obvious.

1 files changed, 17 insertions, 6 deletions

diff --git a/bin/autoheader.in b/bin/autoheader.in
index 7d07fad1..bd824ea3 100644
--- a/bin/autoheader.in
+++ b/bin/autoheader.in
@@ -273,13 +273,24 @@ $out->close;
 {
   foreach my $template ("$tmp/config.hin", @config_templates)
     {
-      my $in = new Autom4te::XFile ($template, "<");
-
-      while ($_ = $in->getline)
+      if ($template =~ /["#\$&'()*;<>?[\\\^`|]/)
+	{
+	  msg 'syntax', "warning: header template has shell metacharacters: $template";
+	}
+      elsif (! -r $template)
+	{
+	  msg 'syntax', "warning: cannot read $template: $!";
+	}
+      else
 	{
-	  my ($sym) = /^\#\s*\w+\s+(\w+)/
-	    or next;
-	  delete $symbol{$sym};
+	  my $in = new Autom4te::XFile ($template, "<");
+
+	  while ($_ = $in->getline)
+	    {
+	      my ($sym) = /^\#\s*\w+\s+(\w+)/
+		or next;
+	      delete $symbol{$sym};
+	    }
 	}
     }