x11-misc/xcalendar: pass -std=gnu89

Does not build with `clang -std=c2x`, early workaround for
when this will become a default.

(not actively hunting for these, merely revisiting packages
previously looked at for clang16 even if not an issue "yet").

Signed-off-by: Ionen Wolkens <ionen@gentoo.org>

x11-misc/xcalendar/xcalendar-4.0-r3.ebuild

@@ -42,6 +42,7 @@ src_prepare() {
 
 src_compile() {
 	tc-export CC
+	append-cflags -std=gnu89 # old codebase, incompatible with c2x
 
 	if use motif; then
 		append-cppflags $($(tc-getPKG_CONFIG) --cflags x11 xmu xt || die)