Build system refactoring

Reorganise the makefiles so subprojects can be more separately used and
maintained.  Still kept the single toplevel Makefile and config.mk.

Subproject Dir.mk is expected to provide all-X, check-X, clean-X and
install-X targets where X is the subproject name and it may use generic
make variables set in config.mk, like CFLAGS_ALL and CC, or subproject
specific variables like X-cflags.