Add link dependency on shlwapi.lib during build configuration instead of #pragma comment statement. Use this library also on MinGW.

Expose as few system variables as possible through public interface.
Perform STRIP_FLAGS_HELP test using CMake instead of Bash.
Change file path separator used by gflags_reporting.cc to backslash on Windwos.

Remove build tool related files, operating system packages, and move unit tests to separate subdirectory.