haskell: make generic builder follow compiler’s shared config

enableShared in generic-builder.nix should default to what the GHC
compiler was compiled with. Add a passthru to all of the GHC compilers
to hold the value of enableShared. If enableShared is not set in the
GHC we just use false as the default value for enableSharedLibraries.

Note: I may have missed some compilers. Only GHC & GHCJS are covered
by this commit but this shouldn’t break evaluation of anything else.

pkgs/development/compilers/ghcjs/base.nix

@@ -179,6 +179,8 @@ in mkDerivation (rec {
     # let us assume ghcjs is never actually cross compiled
     targetPrefix = "";
 
+    enableShared = true;
+
     inherit stage1Packages;
     mkStage2 = stage2 {
       inherit ghcjsBoot;