configure.ac: properly evaluate ac_test_CFLAGS

Since autoconf 2.69 and
https://git.savannah.gnu.org/cgit/autoconf.git/commit/?id=76754e04fce5f6a7701bec57b057020585df2ae3


ac_test_CFLAGS is set to ${CFLAGS+y} instead of ${CFLAGS+set}.

Just test that ac_test_CFLAGS is not empty, to support both cases.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

configure.ac

@@ -120,7 +120,7 @@ AC_SEARCH_LIBS([clock_gettime], [rt posix4],
 AC_CANONICAL_HOST
 
 # Store this value because ac_test_CFLAGS is overwritten by AC_PROG_CC
-if test "$ac_test_CFLAGS" != set ; then
+if ! test "$ac_test_CFLAGS" ; then
   bird_cflags_default=yes
 fi