Use ```_static``` suffix in filename of static library on Windows to distinguish it from the import library.

Add Bazel build support

Define NO_THREADS when building without threads

Previously NOTHREADS was being defined but the code
in mutex.h checks for NO_THREADS.

Resolve some compiler warnings

MSVC 2015 already defines vsnprintf() and va_copy().

Fix build with MS VS 2015 RC

Earlier version, such as CMake 2.8.11.2 in particular, have incomplete or faulty support of target_include_directories generator expressions. Moreover, this allows us to assume that the command target_include_directories is available.

The gflags_TARGET variable contains the name of the originally exported import target. The "gflags" import target contains a copy of the relevant properties of this import target created by CMake. The gflags_LIBRARIES variable can be used to be consistent with other packages, but is otherwise no longer needed. In most cases, "target_link_libraries(foo gflags)" should be used.