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.

Fix previous change of FindThreadsCxx.cmake by copying needed source file from CMake installation, changing the file name extension only.

Fix case in CMake module includes.

* C language definition is needed for CheckForPthreads.c

Adding gitignore for generated files