Having just one depfile (.depend) means it has to be fully regenerated
on every change, and it can't be done in parallel.
After this change the rebuild time after touching a single test file has
gone from 5.0 to 0.9 seconds. (make -j12)
Build of tests from clean has gone from 8.1 to 5.5s.
We could go even further and have one depfile per c-file, but this felt
like a simple first step giving a nice improvement.
