This allows users to have a collection of build variables set without
needing to specify them at build time every time.

apparently the double colon rule dependencies affect only the
specific rule-set.

So we don't get local modifications or extra files in the tarball.

Tracks now probler header file dependencies and command line parameters
used to build files. E.g. changing CFLAGS rebuild all C-files. And changing
version rebuild now the files where it's used.