A few portability issues with the use of dirent for validating forcing file paths in model engine.

[hellkite500]

In this directory iteration loop in the FormulationManager, there are two potential issues.

First, according to the documentation, d_type is not guaranteed to be a member of the dirent struct on all systems, so this requires a macro guard using #ifdef _DIRENT_HAVE_D_TYPE to prevent compiler errors on such systems.

Second, d_type is not guaranteed to be provided by all file systems. As such, a fallback to stat is required for cases when DT_UNKOWN is returned. Note that reading the dirent struct is more efficient, and should be tried first, but this fallback is needed as well.