- Add missing gflags threading dependency.
- Also add optional gflags Shlwapi dependency on Windows.
- Add check_cxx_source_compiles() with CMAKE_BUILD_TYPE to fix
  namespace test builds on Windows with NMake generators.
- Add namespace regex on gflags.h when check_cxx_source_compiles()
  fails on Windows using Visual Studio generator.

Change-Id: I6570c566e5a29c665d621a54935c16d16bac7117