[FEATURE] clang warning level

 * customised clang warning levels and fixed some first issues reported by clang
 * each library now uses `SYSTEM` includes to instruct the compiler from where warnings should be reported
 * renamed some internal variables to match the respective target names (e.g., `OPENMS_INCLUDE_DIRECTORIES -> OpenMS_INCLUDE_DIRECTORIES`) to prepare easier target exports (see #676)
 * fixes #652