Clang does not recognize some optimization flags.

Namely, -fno-tree-vrp and -fno-guess-branch-possibility.
Improve how we check what the compiler is.

Using the check shown at
http://stackoverflow.com/questions/10046114/in-cmake-how-can-i-test-if-the-compiler-is-clang.
Merge pull request #63 from chrisdembia/clangwarnings

Clang warnings