[libvirt] [libvirt-glib] build: allow building with newer glibc-headers and -O0

[Christophe Fergeau]

Commit log and autoconf snippet written by Eric Blake for libvirt.

glibc 2.15 (on Fedora 17) coupled with explicit disabling of
optimization during development dies a painful death:

In file included from /usr/include/limits.h:27:0,
                 from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/include/limits.h:169,.
                 from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/include/syslimits.h:7,
                 from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/include/limits.h:34,
                 from util/bitmap.c:26:
/usr/include/features.h:314:4: error: #warning _FORTIFY_SOURCE requires compiling with optimization (-O) [-Werror=cpp]
cc1: all warnings being treated as errors

Work around this by only conditionally defining _FORTIFY_SOURCE,
in the case where glibc can actually use it.  The trick is using
AH_VERBATIM instead of AC_DEFINE.

* m4/virt-compile-warnings.m4 (LIBOSINFO_COMPILE_WARNINGS): Squelch
_FORTIFY_SOURCE when needed to avoid glibc #warnings.
---
 m4/virt-compile-warnings.m4 |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/m4/virt-compile-warnings.m4 b/m4/virt-compile-warnings.m4
index de542cc..99d214e 100644
--- a/m4/virt-compile-warnings.m4
+++ b/m4/virt-compile-warnings.m4
@@ -84,8 +84,13 @@ AC_DEFUN([LIBVIRT_GLIB_COMPILE_WARNINGS],[
     gl_WARN_ADD([-Wframe-larger-than=4096])
 
     # Use improved glibc headers
-    AC_DEFINE([_FORTIFY_SOURCE], [2],
-      [enable compile-time and run-time bounds-checking, and some warnings])
+    AH_VERBATIM([FORTIFY_SOURCE],
+    [/* Enable compile-time and run-time bounds-checking, and some warnings,
+        without upsetting newer glibc. */
+     #if defined __OPTIMIZE__ && __OPTIMIZE__
+     # define _FORTIFY_SOURCE 2
+     #endif
+    ])
 
     # Extra special flags
     dnl -fstack-protector stuff passes gl_WARN_ADD with gcc
-- 
1.7.10.2