WIP: Update CMake to allow for prebuild dependencies

[mathomp4]

WORK IN PROGRESS

This is a work in progress. It works for me but I've currently broken the regular build. Nuts.

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Closes #128

This PR is to allow prebuilt functional-fortran, h5fortran, and json-fortran to be used with neural-fortran.

[milancurcic]

Sorry for the delay. This works well for me. I tested with system-provided builds of HDF5, both serial and OpenMPI. I also tested with a custom serial Intel build of HDF5, that builds fine too.

The only incorrect thing I found is that even when the dependencies are found the CMake reports messages like

-- h5fortran not found, fetching from GitHub

but doesn't proceed to download them. I'm confused by this because it doesn't look from e.g. cmake/functional.cmake that that should happen.

[mathomp4]

@milancurcic Hmm. I'll take a look.

Though I still think I might need to consult with @scivision on h5fortran. Or, I suppose, maintain a fork of it for now.