[cmake] Support enabling -Werror in the OpenMP runtime CMake build and

clean up the build.

This disables all of the Clang warnings that fire for me when building
libomp.so on Linux with a recent Clang binary. Lots of these should
probably be fixed, but I want to at least get the build warning-clean
and make it easy to keep that way.

I also switched a bunch of the warnings that are used both for C and C++
compiles to check the flag with C compilation test.

Differential Revision: http://reviews.llvm.org/D11253

llvm-svn: 242604