coverage: distcheck-hook to catch missing/outdated *.m4 files Related to automake bug#9037. * tests/distcheck-missing-m4.test: New test. * tests/distcheck-outdated-m4.test: Likewise. * tests/distcheck-hook-m4.am: New data file, used by the new tests. * tests/Makefile.am (distcheck-m4-missing.log, distcheck-m4-outdated.log): Depend on it. (EXTRA_DIST): Distribute it. (TESTS): Add the new tests.