minimal_header: add a forward declaration and pure virtual class

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>
extractor: current state

- skip system headers
- add "isDefinition" to the ast matcher filters aus the "double-match"
- adding vim folds... sorry non-vim folks ;-)
- adding gcc-4.9 specific compiler option to remove a warning

Signed-off-by: Martin Zenzes <martin.zenzes@dfki.de>