gcc-*: homogenize and cleanup expressions without a rebuild

Looks scary but it is a noop.

pkgs/development/compilers/gcc/7/default.nix

@@ -39,8 +39,6 @@ with builtins;
 
 let version = "7.3.0";
 
-    enableParallelBuilding = true;
-
     inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 
     patches =
@@ -344,10 +342,13 @@ stdenv.mkDerivation ({
         "-Wl,-rpath-link,${libcCross.out}/lib"
     ]));
 
-  passthru =
-    { inherit langC langCC langObjC langObjCpp langFortran langGo version; isGNU = true; };
+  passthru = {
+    inherit langC langCC langObjC langObjCpp langFortran langGo version;
+    isGNU = true;
+  };
 
-  inherit enableParallelBuilding enableMultilib;
+  enableParallelBuilding = true;
+  inherit enableMultilib;
 
   inherit (stdenv) is64bit;
 
@@ -371,6 +372,7 @@ stdenv.mkDerivation ({
     platforms =
       stdenv.lib.platforms.linux ++
       stdenv.lib.platforms.freebsd ++
+      stdenv.lib.platforms.illumos ++
       stdenv.lib.platforms.darwin;
   };
 }