debuild: Make 'make check' work, cosmetic changes.
I suggest to merge this right after the 2.15.0 release branch !575 (merged) has been merged and 2.15.0 has been tagged.
This fixes the glitches preventing make check
to work in the Debian/Ubuntu OBS builds, so that those builds now run our full regression test suite. Only the toplevel Makefile and the debuild rules file are affected. Tested on the OBS and in local builds on Linux, Mac and Windows.