build: fix circular header inclusion with other packages

This commit renames lib/setup.h to lib/curl_setup.h and
renames lib/setup_once.h to lib/curl_setup_once.h.

Removes the need and usage of a header inclusion guard foreign
to libcurl. [1]

Removes the need and presence of an alarming notice we carried
in old setup_once.h [2]

----------------------------------------

1 - lib/setup_once.h used __SETUP_ONCE_H macro as header inclusion guard
    up to commit ec691ca3 which changed this to HEADER_CURL_SETUP_ONCE_H,
    this single inclusion guard is enough to ensure that inclusion of
    lib/setup_once.h done from lib/setup.h is only done once.

    Additionally lib/setup.h has always used __SETUP_ONCE_H macro to
    protect inclusion of setup_once.h even after commit ec691ca3, this
    was to avoid a circular header inclusion triggered when building a
    c-ares enabled version with c-ares sources available which also has
    a setup_once.h header. Commit ec691ca3 exposes the real nature of
    __SETUP_ONCE_H usage in lib/setup.h, it is a header inclusion guard
    foreign to libcurl belonging to c-ares's setup_once.h

    The renaming this commit does, fixes the circular header inclusion,
    and as such removes the need and usage of a header inclusion guard
    foreign to libcurl. Macro __SETUP_ONCE_H no longer used in libcurl.

2 - Due to the circular interdependency of old lib/setup_once.h and the
    c-ares setup_once.h header, old file lib/setup_once.h has carried
    back from 2006 up to now days an alarming and prominent notice about
    the need of keeping libcurl's and c-ares's setup_once.h in sync.

    Given that this commit fixes the circular interdependency, the need
    and presence of mentioned notice is removed.

    All mentioned interdependencies come back from now old days when
    the c-ares project lived inside a curl subdirectory. This commit
    removes last traces of such fact.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 54162b3b5..7e98d115b 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -27,11 +27,11 @@ AUTOMAKE_OPTIONS = foreign nostdinc
 # might possibly already be installed in the system.
 #
 # $(top_builddir)/include/curl for generated curlbuild.h included from curl.h
-# $(top_builddir)/include for generated curlbuild.h included from lib/setup.h
+# $(top_builddir)/include for generated curlbuild.h inc. from lib/curl_setup.h
 # $(top_srcdir)/include is for libcurl's external include files
 # $(top_builddir)/lib is for libcurl's generated lib/curl_config.h file
 # $(top_builddir)/src is for curl's generated src/curl_config.h file
-# $(top_srcdir)/lib is for libcurl's lib/setup.h and other "borrowed" files
+# $(top_srcdir)/lib for libcurl's lib/curl_setup.h and other "borrowed" files
 # $(top_srcdir)/src is for curl's src/tool_setup.h and "curl-private" files
 
 AM_CPPFLAGS = -I$(top_builddir)/include/curl \