CMake: added autogeneration of build flags in config.h header file

CMake: added autogeneration of build flags in config.h header file

now all build flags are automatically exported to the auto_generated
config.h file.

Note: config.h is no created in two passes:
1. pass: #cmakedefine macros are created for each build flag
2. pass: #cmakedefine macros are evaluated

Signed-off-by: Timo Lotterbach <timo.lotterbach@bmw-carit.de>