--- a/sys-devel/flex/flex-2.5.39-r1.ebuild
+++ b/sys-devel/flex/flex-2.5.39-r1.ebuild
@@ -22,6 +22,10 @@
 	nls? ( sys-devel/gettext )
 	test? ( sys-devel/bison )"
 
+src_prepare() {
+	epatch "${FILESDIR}"/${PN}-2.5.39-bison-test-fixes-Do-not-use-obsolete-bison-construct.patch
+}
+
 src_configure() {
 	use static && append-ldflags -static