fcm make: build: more intelligence on external dependencies

[matthewrmshin]

Suppose we have something like this in a Fortran source file:

use an_external_mod, only: a_subroutine

where an_external_mod is not part of the source tree. Normally, we'll just do:

build.prop{no-dep.f.module} = an_external_mod

However, fcm make will have no idea if an_external_mod has been modified in an incremental build or not. If the location of an_external_mod.mod and an_external_mod.o can be provided, it should be possible for the build system to determine if it needs to re-compile and/or re-link any dependent object files and executables.