ghc: fix testsuite, compile problem

The structure of testsuite is different between the stable one and the devel one(--devel)
If install ghc with --tests options
it will error with "testsuite" not found message.
The cc setting is set at `./configutre ... --with-gcc=... `. Using pure clang will fail some ghc compile because it use `C pre-processor` as a macro processor for haskell compiler while parameters on gcc and clang are different.