build: Add -std=c99 for bootstrapping muon

On older distros the bootstrap step fails because C99 is not the
default C version. Thus explicitly set it.

Signed-off-by: Tokunori Ikegami <ikegami.t@gmail.com>
[dwagner: updated commit message]
Signed-off-by: Daniel Wagner <dwagner@suse.de>

scripts/build.sh

@@ -153,7 +153,8 @@ tools_build_muon() {
 
     pushd "${TOOLDIR}/muon" || exit 1
 
-    CC="${CC}" ninja="${SAMU}" ./bootstrap.sh stage1
+    CC="${CC}" CFLAGS="${CFLAGS} -std=c99" ninja="${SAMU}" \
+        ./bootstrap.sh stage1
 
     CC="${CC}" ninja="${SAMU}" stage1/muon setup        \
         -Dprefix="${TOOLDIR}"                           \