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

Exporting binary tree to CMake registry

This eases integration with other applications making use
of gflags without needing to install it in the system.
find_package looks in the registry for existing software.

Configure and install CMake package configuration file to help find_package locate the gflags package.