diff --git a/pkgs/applications/version-management/rcs/default.nix b/pkgs/applications/version-management/rcs/default.nix
index a829af8aa23..44f72c3f0fe 100644
--- a/pkgs/applications/version-management/rcs/default.nix
+++ b/pkgs/applications/version-management/rcs/default.nix
@@ -12,7 +12,7 @@ stdenv.mkDerivation rec {
 
   doCheck = true;
 
-  NIX_CFLAGS_COMPILE = if stdenv.isDarwin then "-std=gnu99" else null;
+  NIX_CFLAGS_COMPILE = [ "-std=c99" ];
 
   meta = {
     homepage = http://www.gnu.org/software/rcs/;