x11-misc/x2vnc: 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/x2vnc/x2vnc-1.7.2-r2.ebuild

@@ -3,6 +3,8 @@
 
 EAPI=8
 
+inherit flag-o-matic
+
 DESCRIPTION="Control a remote computer running VNC from X"
 HOMEPAGE="https://fredrik.hubbe.net/x2vnc.html"
 SRC_URI="https://fredrik.hubbe.net/x2vnc/${P}.tar.gz"
@@ -31,6 +33,12 @@ PATCHES=(
 	"${FILESDIR}"/${P}-clang16.patch
 )
 
+src_configure() {
+	append-cflags -std=gnu89 # old codebase, incompatible with c2x
+
+	default
+}
+
 src_install() {
 	dodir /usr/share /usr/bin
 	emake DESTDIR="${D}" install