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/4.8/default.nix

@@ -5,6 +5,7 @@
 , langJava ? false
 , langGo ? false
 , profiledCompiler ? false
+, langJit ? false
 , staticCompiler ? false
 , enableShared ? true
 , enableLTO ? true
@@ -212,6 +213,7 @@ stdenv.mkDerivation ({
       langGo
       langObjC
       langObjCpp
+      langJit
       ;
   };