configure: fix compilation on hosts without -fstack-protector-all

Commit a0f291fc has enabled
-fstack-protector-all on all targets, as the configure test is bogus.
GCC only emits a warning and not an error if this option is not
supported, so the configure scripts doesn't detect the problem.

This patch changes the configure script to try the various flags
with -Werror in addition to catch the possible warnings.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

configure

@@ -140,11 +140,14 @@ LDFLAGS="-g $LDFLAGS"
 
 gcc_flags="-Wold-style-declaration -Wold-style-definition -fstack-protector-all"
 cat > $TMPC << EOF
-int main(void) { }
+int main(void) { return 0; }
 EOF
 for flag in $gcc_flags; do
-    if compile_prog "$QEMU_CFLAGS" "$flag" ; then
+    echo $testing $flag
+    if compile_prog "-Werror $QEMU_CFLAGS" "-Werror $flag" ; then
 	QEMU_CFLAGS="$flag $QEMU_CFLAGS"
+    else
+        echo disable $flag
     fi
 done