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.
This is needed for some cabal2nix stuff in getting the version.
Previously we had left out the version but apparently this can cause
problems in some scenarios.
/cc @elvishjerricco @peti @ericson2314
