Add cache database to dependency tracker
On platforms where the Fortran compiler is slow (I'm looking at you xlf) to generate preprocessor output, it would be highly desirable to cache the dependency results, and not reparse the source code of unchanged files. Such an approach would have to account for several preconditions:
- The environment and flags between runs would ideally be unchanged, perhaps excluding known problem cases.
- The source file in question must not have changed, as well as any file it includes.
Please add further conditions to account for.