The MPI external library currently does not provide Fortran bindings for MPI. I observe this on a Linux box with OpenMPI installed and mpicxx in the path and with no MPI_XXX set in my option list. ExternalLibraries/MPI parses the output of mpicxx -showme:link which I verified to happen by inspecting its output and bindings/Configuration/Capabilities/make.MPI.defn.
For the autodetection we'd have to also consider the output of mpif90 -showme:link and likely also mpif77 showme:link in case the F90 interfaces are somewhere other than the (typeless) f77 interfaces.
This can be impossible for the case where we build OpenMPI with the toolkit since OpenMPI will provide either F2008 or F90 bindings depending on the compiler and the libraries for the two have different names. This however is something that is only decided in OpenMPI's own configure script.
We also need to make sure mpi.mod and mpif.h are found by Fortran's "USE" and "INCLUDE" statements respectively.