install back the safety check of includes.test
Compilation of each header file alone, as a safety check, was removed when introducing "#pragma once" because we did not have to check for possible double inclusion. However we still need to compile each header to make sure they are self-contained. * tests/sanity/includes.test: Compile each header. * tests/run.in: Export various compiler and directory flags. * spot/twaalgos/emptiness_stats.hh, spot/misc/mspool.hh, spot/misc/fixpool.hh: Include <spot/misc/common.hh>. * spot/misc/common.hh: Include <cassert>. * NEWS: Mention the fixed headers.