build system: Makefile.in: always include cfgdefs.mk & usrdefs.mk.
    
    The purpose of this commit is twofold; from the user's perspective, this
    provides a consistent experience whenever building a sofort-based project;
    and from the developer's perspective, this provides an easy way to test
    cfgdefs.mk even before the writing and/or integration of cfgdefs.sh.