Thanks @aradi, if you have ideas to propose about how to introduce some flexibility in the fpm manifest, please do so here or on the github issues page!
There is currently no explicit way to use coarrays either. I myself do not use coarrays, so I may be mistaken. I think the only current way to use coarrays in fpm is via a custom compiler command:
fpm --compiler=caf
fpm --compiler="ifort -coarray"