Allow unit testing on Windows

These changes allow testing on Windows using clang.exe.
There are two main changes:

1. Only link to -lm when it actually exists on the system
2. Create basic versions of pthread_create() and pthread_join() for windows. They are not POSIX compliant by any stretch but will allow any existing and future tests to use pthread_create() and pthread_join() for testing interactions of libomp with os threads.

Differential Revision: http://reviews.llvm.org/D20391