BUILD: makefile: fix the "make clean" target on strict bourne shells

BUILD: makefile: fix the "make clean" target on strict bourne shells

As reported by @axinojolais in issue #1217, some older bourne shells do
not expand on braces so some files were not cleaned since the recent
splitting of the contrib/ subdir. Let's fix that by explicitly listing
the patterns to be cleared (which are in much smaller quantity now that
contrib was removed), and for grouping them with their respective dirs.

At some point, some recursive makefiles would probably help there.