x11-misc/dclock: 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/dclock/dclock-2.2.2_p12.ebuild

@@ -45,6 +45,8 @@ src_prepare() {
 }
 
 src_configure() {
+	append-cflags -std=gnu89 # old codebase, incompatible with c2x
+
 	if use xft; then
 		append-cppflags \
 			-DXFT_SUPPORT \