Remove 'program' constraint from C-dependencies

This fixes #1387

Currently, when a library has C-sources and e.g. an executable
depends on that library, then changing the C-sources and running
'-build' does not yield a rebuild of the library.
By adding these dependencies (rather removing the constraint),
it seems to work fine.

Signed-off-by: Christoph Höger <christoph.hoeger@tu-berlin.de>