Merge pull request #140 from schuhschuh/enh-#117-gflags-subproject-support

Merge pull request #140 from schuhschuh/enh-#117-gflags-subproject-support

These changes improve the CMakeLists.txt of the gflags project to make CMake variables configurable via command-line option -D or super-project settings. Moreover, gflags related variables are only added to the CMake cache when the gflags project is configured as a standalone project. When it is a subproject (cf. #117), the super-project is responsible for setting the configuration variables.

This change set also changes the names of the (exported) target names to gflags[_nothreads]_(static|shared) and the CMake (package) configuration variable names to start with an uppercase prefix "GFLAGS_".