gcc: add langJit option for gcclangjit

This option can be used to set the “jit” language which enable the
libgccjit functionality. Also adds a “libgccjit” attr which is gcc
built with just jit enabled.

pkgs/development/compilers/gcc/9/default.nix

@@ -5,6 +5,7 @@
 , langObjCpp ? stdenv.targetPlatform.isDarwin
 , langGo ? false
 , profiledCompiler ? false
+, langJit ? false
 , staticCompiler ? false
 , enableShared ? true
 , enableLTO ? true
@@ -85,7 +86,7 @@ stdenv.mkDerivation ({
 
   inherit patches;
 
-  outputs = [ "out" "man" "info" ] ++ lib.optional langJit "lib";
+  outputs = [ "out" "man" "info" ] ++ stdenv.lib.optional (!langJit) "lib";
   setOutputFlags = false;
   NIX_NO_SELF_RPATH = true;
 
@@ -202,6 +203,7 @@ stdenv.mkDerivation ({
       langGo
       langObjC
       langObjCpp
+      langJit
       ;
   };