diff --git a/dev-util/ftjam/ftjam-2.5.3_rc2-r3.ebuild b/dev-util/ftjam/ftjam-2.5.3_rc2-r3.ebuild
index 806e3bc1fa142..07202ada9f35f 100644
--- a/dev-util/ftjam/ftjam-2.5.3_rc2-r3.ebuild
+++ b/dev-util/ftjam/ftjam-2.5.3_rc2-r3.ebuild
@@ -3,7 +3,7 @@
 
 EAPI=7
 
-inherit toolchain-funcs
+inherit flag-o-matic toolchain-funcs
 
 MY_PV=$(ver_rs 3 "")
 
@@ -33,5 +33,6 @@ src_prepare() {
 	default
 
 	tc-export CC RANLIB
+	append-cflags -std=gnu89 # old codebase, incompatible with c2x
 	export AR="$(tc-getAR) ru" #720706
 }