testing/libmdbx: new aport

libmdbx is an extremely fast, compact, powerful, embedded, transactional key-value database, with permissive license.

It's preparation for patching kopano-core to use libmdbx instead of bsddb. To do that we still need some Python bindings. SWIG and other tools have failed at that task with the libmdbx headers up to now, so it seems it's a little bit more complicated.

The massive patch file adds the test suite that the dev strips when creating the "libmdbx-amalgated" archives for release. Alternatively, one could theoretically download the source archive, but making a reasonable library out of that source archive is more complex and work intensive than patching the amalgated version and adding the test suite to it. So that's what I did.

Currently WIP because debugging for ppc64le is needed.

Tested on ppc64le and issue reported upstream. Ready for merging.