Fix the LLVM flag

Apparently Cabal normalizes the flags to lower case, and the
lookup for "LLVM" does nothing. Commit
2972b75a1896fc8d0db1e4d442f6aa5b2e1caf8e isn't wrong, but the problem
is that it ever gets to the Nothing case. The flag should always
be there.
Merge pull request #422 from melted/llvm

Fix the LLVM flag