rcs: Fix build on gcc 5

http://hydra.nixos.org/build/32624218

pkgs/applications/version-management/rcs/default.nix

···
 
   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/;