net-misc/rsync: build w/ -std=gnu17 (for now)

There's fixes upstream but I'd rather wait for the next release unless
someone else wants to backport.

Closes: https://bugs.gentoo.org/943745
Signed-off-by: Sam James <sam@gentoo.org>

net-misc/rsync/rsync-3.3.0-r1.ebuild

@@ -108,6 +108,9 @@ src_prepare() {
 }
 
 src_configure() {
+	# Should be fixed upstream in next release (>3.3.0) (bug #943745)
+	append-cflags -std=gnu17
+
 	local myeconfargs=(
 		--with-rsyncd-conf="${EPREFIX}"/etc/rsyncd.conf
 		--without-included-popt