Clang warnings

Clang doesn't recognize -fno-tree-vrp -fno-guess-branch-probability, and emits warnings. These flags are only provided if Clang is NOT the compiler.

Also, I change the way how we check if Clang is the compiler. It may make sense to make more similar changes. We care if Clang is the CXX compiler, I think, not if it's the C compiler.

The way I check the compiler for Clang is as shown here: http://stackoverflow.com/questions/10046114/in-cmake-how-can-i-test-if-the-compiler-is-clang