Hello,

This patch enhances config/zlib.m4 to introduce an extra option
--with-libz-prefix which allows us to provide the location of
the zlib library we want to use during the build.

config/ChangeLog:

        * zlib.m4 (AM_ZLIB): Add --with-libz-prefix option support.

I didn't see any file in the GCC project that uses this macro,
so for the GCC repository, the change to zlib.m4 is it. But
I am also attaching to this email a copy of the patch that
will be applied to the binutils-gdb.git repository, with all
configury using this macro being re-generated - mostly for info,
also as a heads-up to both binutils and GDB.

This was tested by regenerating all autoconf/automake files in
the binutils-gdb project, and rebuilding GDB, using the following
combinations:

  --with-zlib (system zlib used)
  --with-libz-prefix=/zlib/prefix (specific zlib linked in)
  --with-zlib --with-libz-prefix=/zlib/prefix (specific zlib linked in)

  --without-zlib (zlib support turned off)
  --without-zlib --with-zlib-prefix (zlib support turned off)

  --with-zlib (no system zlib available, configure fails with expected error)
  --with-zlib --with-libz-prefix=/invalid/zlib/prefix
              (no system zlib, configure fails with same error)

OK to commit?

Thank you!
-- 
Joel

---
 config/zlib.m4 | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/config/zlib.m4 b/config/zlib.m4
index b017499..1581124 100644
--- a/config/zlib.m4
+++ b/config/zlib.m4
@@ -9,8 +9,15 @@ AC_DEFUN([AM_ZLIB],
     [], [with_zlib=auto])
 
   if test "$with_zlib" != "no"; then
-    AC_SEARCH_LIBS(zlibVersion, z, [AC_CHECK_HEADERS(zlib.h)])
-    if test "$with_zlib" = "yes" -a "$ac_cv_header_zlib_h" != "yes"; then
+    AC_LIB_HAVE_LINKFLAGS([z], [], [#include "zlib.h";],
+                          [int ret = deflateInit(NULL, 1);])
+    if test "$HAVE_LIBZ" = "yes"; then
+      # Make sure HAVE_ZLIB_H gets defined.  Some code uses that over
+      # HAVE_LIBZ, since initially we were only defining HAVE_ZLIB_H.
+      AC_CHECK_HEADERS(zlib.h)
+      LIBS="$LIBS $LIBZ"
+    fi
+    if test "$with_zlib" = "yes" -a "$HAVE_LIBZ" != "yes"; then
       AC_MSG_ERROR([zlib (libz) library was explicitly requested but not found])
     fi
   fi
-- 
1.9.1