cvs: fix CFLAGS only on darwin

Fixes build for pkgsCross.x86_64-netbsd.netbsd.sys

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

···
     "AR=${stdenv.cc.targetPrefix}ar"
   ];
 
-  env = lib.optionalAttrs stdenv.cc.isClang {
+  env = lib.optionalAttrs (stdenv.isDarwin && stdenv.cc.isClang) {
     NIX_CFLAGS_COMPILE = "-Wno-implicit-function-declaration";
   };