Output mepo status on build

[mathomp4]

An interesting thought via @viral211. It would be good to capture the mepo state of GEOSgcm (etc.) if built using mepo. This should be doable in CMake:

1. If .mepo/ exists, we know mepo was used
2. If mepo command exists then...
3. Run mepo status --hashes capture the output and put it in a file we install and then
   copy into the run directory. Maybe a dotfile?

I'm not sure what I'd want to call the file. Maybe .mepo_status? Hmm.

First step, get the code working, then I can bother @sdrabenh.