Build: Include common.mk in all Makefile.am files.

This then requires a couple other changes:

• Any file that sets MAINTAINERCLEANFILES can drop that line, unless it's adding something to it. common.mk always adds Makefile.in to that variable.

• lib/cib/Makefile.am doesn't need to set libcib_la_CPPFLAGS because it's not adding anything on top of the default AM_CPPFLAGS.

• unittest.mk always gets included after common.mk now, so it can just append the test-specific things it needs to AM_CPPFLAGS.