Some necessary refactoring.

There's now three distinct actions:

 - Importing some system library: _gmock_find_and_import_from_system
 - Importing from an external project: _add_external_project_with_gmock_cflags
 - Finding a source dir: _gmock_find_src_dirs_from_include_paths

Also refactored CXX flag checking into _gmock_add_cxx_flag.

Finally, Use standard FindPackageHandleStandardArgs and FindPackageMessage