Don't include deps during 'make clean'.
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.