cpp: Use portable off_t size_t

* configure.ac: Configure cpp data.h.in
* lang/cpp/src/Makefile.am: Generate data.h
* lang/cpp/src/data.cpp, lang/cpp/src/data.h: Use portable
types.
* lang/qt/src/Makefile.am: Include build dir.
* lang/qt/tests/makefile.am: Include build dir.

These kind of patches have been around for a while, IMO this
should not create an ABI incompatbility for cases where
it already works because the types should be the same
so I think this is not an interface break.

• GnuPG-Bug-Id: T3996