ghc 7.8.2 with --devel

ghc 7.8 uses clang internally.
clang is used to build the ghc 7.8 on mavericks 64 bit, and gcc is used on
older OS and 32 bit builds.
Also advise to use cabal-install in the caveat.

Closes #29084.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>