Simplify configure.ac.in by using XDT_VERSION_INIT

Tried to follow xfdesktop!97 (merged), for getting rid of configure.ac.in preprocessing, but failed for thunarx_version_api and thunar_debug_default (I suppose it is required to get rid of all m4_define to prevent preprocessing ?)

@kelnos, if you have further hints, they would be welcome

I suppose fine to at least push this minor simplification 🙂