Properly handle dependencies for deleted header files

When a header file is deleted, make will complain "No rule to make target <old header file>". This often breaks incremental build completely unnecessary.

This can be fixed by adding dummy rules for all header files like this:
<header file>:

gcc can create such rules by itself by using -MP in the generated make dependency files, but that feature is not available for any other compiler, so I opted not to use it, in favour of a solution that works on all platforms.