python: Link 'data.h' and 'config.h' into the builddir.

* lang/python/Makefile.am: Link to the files.
* lang/python/gpgme.i: Update path.
* lang/python/setup.py.in: Do not add the top builddir to the include
path.

To make it easy to build the subpackage using standard tools without
altering environment or CFLAGS, symlink the required artifacts from
source tree into subpackage directory when preparing sources.

• Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>