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

The OS_WINDOWS #define has previously been used in gflags.cc before the CMake migration.

Copied from CMake master branch.

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.