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New substitution: top_build_prefix


From: Ralf Wildenhues
Subject: New substitution: top_build_prefix
Date: Thu, 8 Nov 2007 21:05:24 +0100
User-agent: Mutt/1.5.13 (2006-08-11)

Hello Autoconf patchers,

We have hit another bug in HEAD Libtool, for which we could use help
from Autoconf.

This is the setting: a third-party package (GraphicsMagick) that uses
libltdl in nonrecursive mode in a nonrecursive Makefile[1].  In this
Makefile, the library is given as
  noinst_LTLIBRARIES = ltdl/libltdlc.la

however the dependency is given upon
  LIBLTDL = ${top_builddir}/ltdl/libltdl.la

and in this case, top_builddir is `.'.  LIBLTDL is computed as a
substitution of @LIBLTDL@ at configure time.

Now, AIX make (and others) fail to identify `./file' with `file' so the
build fails.  It would be nice if Autoconf also substituted a variable
top_build_prefix that contained of zero or more runs of `../' and
otherwise behaved like top_builddir.  The naming is not coincidental:
config.status already computes this value, it just doesn't make it
available.

This would make writing these kinds of things much easier also in
Automake, which has lots of special-cases of the kind
  if ($directory ne '')
    {
      $object = $directory . '/' . $object;
    }

Note that top_build_prefix cannot replace top_builddir: the former
requires the user _not_ to add a slash as separator.

[1] Yes, this is yet another instance where nonrecursive makefiles are
more difficult to realize for non-GNU makes.

Cheers,
Ralf

    New config files output variable `top_build_prefix'.
    
    * lib/autoconf/status.m4 (_AC_OUTPUT_FILE): Substitute
    `top_build_prefix'.
    * doc/autoconf.texi (Preset Output Variables): Document it.
    * NEWS: Update.
    Report by Bob Friesenhahn.

diff --git a/NEWS b/NEWS
index 77cd6f5..854c54a 100644
--- a/NEWS
+++ b/NEWS
@@ -13,6 +13,8 @@ GNU Autoconf NEWS - User visible changes.
      Further, for config headers, the total size of values is not limited by
      the POSIX length limit of text lines any more, only each single line.
 
+** New config variable `top_build_prefix'.
+
 ** Autoconf is now licensed under the General Public License version 3
    or later (GPLv3+).  As with earlier versions, the license includes
    an exception clause so that you may release a configure script
diff --git a/doc/autoconf.texi b/doc/autoconf.texi
index 9025359..421056e 100644
--- a/doc/autoconf.texi
+++ b/doc/autoconf.texi
@@ -2436,6 +2436,16 @@ The relative name of the top level of the current build 
tree.  In the
 top-level directory, this is the same as @code{builddir}.
 @end defvar
 
address@hidden top_build_prefix
address@hidden top_build_prefix
+The relative name of the top level of the current build tree with final
+slash if nonemtpy.  This is the same as @code{top_builddir}, except that
+it contains of zero of more runs of @code{../}, so it should not be
+appended with a slash for concatenation.  This helps for @command{make}
+implementations that otherwise do not treat @file{./file} and @file{file}
+as equal in the toplevel build directory.
address@hidden defvar
+
 @defvar abs_top_builddir
 @ovindex abs_top_builddir
 Absolute name of @code{top_builddir}.
diff --git a/lib/autoconf/status.m4 b/lib/autoconf/status.m4
index 350d370..4412df0 100644
--- a/lib/autoconf/status.m4
+++ b/lib/autoconf/status.m4
@@ -622,6 +622,7 @@ dnl configure_input is a somewhat special, so we don't call 
AC_SUBST_TRACE.
 s&@configure_input@&$configure_input&;t t
 dnl During the transition period, this is a special case:
 s&@top_builddir@&$ac_top_builddir_sub&;t t[]AC_SUBST_TRACE([top_builddir])
+s&@top_build_prefix@&$ac_top_build_prefix&;t 
t[]AC_SUBST_TRACE([top_build_prefix])
 m4_foreach([_AC_Var], [srcdir, abs_srcdir, top_srcdir, abs_top_srcdir,
                        builddir, abs_builddir,
                        abs_top_builddir]AC_PROVIDE_IFELSE([AC_PROG_INSTALL], 
[[, INSTALL]])AC_PROVIDE_IFELSE([AC_PROG_MKDIR_P], [[, MKDIR_P]]),




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