If $(CACHE_PROJECT_MK) AKA .cache-project.mk already exists but needs
to be remade because of dependencies, it is included, its content
read, then the content cached again to the same file. That way stale
variables can leak into the refreshed cache.
This commit fixes the situation by removing it, then invoking make
again to remake it, thereby clearing the variable database and
forcing make to fill the variables afresh.
Signed-off-by: Jan Lindemann <jan@janware.com>