If a source file moves (as in the previous commit), any stale
dependencies will block execution of 'make' because they
define a dependency relationship to an non-existant file.

The obvious thing to do is 'make clean' to get rid of the
dependency files, but of course that fails the same way.
To support this recovery, skip the include directive if
the GNU make variable MAKECMDGOALS is 'clean', exempting
this target from dependency checks.

Based on an example in the GNU make manual.