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[libvirt] [PATCH] build: avoid warnings on older gcc

The previous patch pulled in a newer version of stat-time.h from
gnulib, which tries to suppress some stupid gcc warnings (see
<http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54113>) by exploiting
a feature of newer gcc.  But we still aim to compile on RHEL 6.3,
with gcc 4.4.6 (not to mention even older devel platforms like
RHEL 5), which fails to build thanks to our development -Werror.

It took me a while to figure out how to make our set of warnings
smaller on older gcc without losing the benefit of the warnings
when using newer gcc (such as the one on Fedora 17), but this
should do the trick.

* m4/virt-compile-warnings.m4 (LIBVIRT_COMPILE_WARNINGS): Avoid
warnings that gnulib can't silence on older gcc.

If I don't get a review by tomorrow, I will go ahead and push this
under the build-breaker rule.

 m4/virt-compile-warnings.m4 | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/m4/virt-compile-warnings.m4 b/m4/virt-compile-warnings.m4
index 9dee000..702506d 100644
--- a/m4/virt-compile-warnings.m4
+++ b/m4/virt-compile-warnings.m4
     # Gnulib's stat-time.h violates this
     dontwarn="$dontwarn -Waggregate-return"

+    # Gnulib uses '#pragma GCC diagnostic push' to silence some
+    # warnings, but older gcc doesn't support this.
+    AC_CACHE_VAL([lv_cv_gcc_pragma_push_works], [
+      save_CFLAGS=$CFLAGS
+      CFLAGS=-Wunknown-pragmas
+        #pragma GCC diagnostic push
+        #pragma GCC diagnostic pop
+      ]])],
+      [lv_cv_gcc_pragma_push_works=yes],
+      [lv_cv_gcc_pragma_push_works=no])
+      CFLAGS=$save_CFLAGS])
+    if test $lv_cv_gcc_pragma_push_works = no; then
+      dontwarn="$dontwarn -Wmissing-prototypes"
+      dontwarn="$dontwarn -Wmissing-declarations"
+    fi
     # We might fundamentally need some of these disabled forever, but
     # ideally we'd turn many of them on
     dontwarn="$dontwarn -Wfloat-equal"

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