[libclang/python] Use relative import for enumerations
[XDRESS-ONLY] [libclang/python] Hack libclang to be 3.2 compatible

This commit is to be used solely in the xdress copy of libclang.
It allows us to use a new, modified version of libclang with an
installed older version of clang.  This commit makes it work for
clang 3.2; it is likely still broken for clang 3.3 in particular.