Ninja: Add explicit preprocessing step for Fortran

All Fortran sources need to be preprocessed before any source may be
compiled so that module dependencies can be (later) extracted.  Factor
out an explicit preprocessing step preceding compilation.  Use Ninja
depfile dependencies on the preprocessing step and then compile the
already-preprocessed source with a separate build statement that depends
explicitly only on the preprocessor output.  Later we will insert
dynamic discovery of module dependencies between these steps.