build: avoid -Wno-format on new-enough gcc

Commit c579d6b3 added a sledgehammer to silence spurious warnings from
gcc 4.2, but in the process, it also silenced useful warnings from
gcc 4.3 through 4.5.  As a result, a bug slipped in to commit 0caccb58.

Tested with FreeBSD (gcc 4.2.1), RHEL 6.3 (gcc 4.4), and F17 (gcc 4.7.2),
where the former didn't trip on spurious warnings, and where the latter
two detected a revert of 2b804cfa.

* m4/virt-compile-warnings.m4 (-Wno-format): Probe for the actual
spurious message, to once again allow gcc 4.4 to use -Wformat.
(cherry picked from commit 814a8dea)

m4/virt-compile-warnings.m4

@@ -60,6 +60,23 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     # gcc 4.4.6 complains this is C++ only; gcc 4.7.0 implies this from -Wall
     dontwarn="$dontwarn -Wenum-compare"
 
+    # gcc 4.2 treats attribute(format) as an implicit attribute(nonnull),
+    # which triggers spurious warnings for our usage
+    AC_CACHE_CHECK([whether gcc -Wformat allows NULL strings],
+      [lv_cv_gcc_wformat_null_works], [
+      save_CFLAGS=$CFLAGS
+      CFLAGS='-Wunknown-pragmas -Werror -Wformat'
+      AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+        #include <stddef.h>
+        static __attribute__ ((__format__ (__printf__, 1, 2))) int
+        foo (const char *fmt, ...) { return !fmt; }
+      ]], [[
+        return foo(NULL);
+      ]])],
+      [lv_cv_gcc_wformat_null_works=yes],
+      [lv_cv_gcc_wformat_null_works=no])
+      CFLAGS=$save_CFLAGS])
+
     # Gnulib uses '#pragma GCC diagnostic push' to silence some
     # warnings, but older gcc doesn't support this.
     AC_CACHE_CHECK([whether pragma GCC diagnostic push works],
@@ -117,7 +134,7 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
     # ATTRIBUTE_FMT_PRINT, which causes -Wformat failure on our
     # intentional use of virReportError(code, NULL).
     gl_WARN_ADD([-Wno-format-nonliteral])
-    if test $lv_cv_gcc_pragma_push_works = no; then
+    if test $lv_cv_gcc_wformat_null_works = no; then
       gl_WARN_ADD([-Wno-format])
     fi