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

llvm-svn: 270464