clean up cmake, make finding LLVM includes more reliable on different platforms

git-svn-id: https://roadrunnerlib.googlecode.com/svn/branches/develop@1084 c0027639-9190-0b57-01eb-f09a03790dae
Merge branch 'gdevelop' into gfeature-llvm

git-svn-id: https://roadrunnerlib.googlecode.com/svn/branches/feature-llvm@1085 c0027639-9190-0b57-01eb-f09a03790dae