xapian-bridge C port

endlessm/eos-sdk#2463
Those variables come from GLib. I found a reference from them in the GTest documentation, but I can't find an online link to it right now... Basically they're needed to instruct the test runner which directories are `srcdir` and `builddir`, so that things like `g_test_get_dir()` work correctly.
If you grep e.g. GTK you can see how other test suites do pretty much the same.