If GAP provides GMP_PREFIX, use that

[fingolfin]

This way if a user has GAP installed via a package distribution in
a "non-standard" location (e.g. Homebrew on macOS, maybe also Conda),
then CaratInterface will be able to find GMP.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.94%. Comparing base (49b1896) to head (74e7d1d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   79.65%   78.94%   -0.72%     
==========================================
  Files           3        3              
  Lines         762      741      -21     
==========================================
- Hits          607      585      -22     
- Misses        155      156       +1     

see 2 files with indirect coverage changes

[fingolfin]

@gaehler would you please consider merging this patch and making a release? It helps macOS users in particular quite a bit, but also people who installed GAP via a package manager (on Debian/Ubuntu/Fedora/...)

(The CI failures against GAP 4.11 are not caused by this PR, but rather by the unfortunate fact that in order to get IO 4.8.0 installed into GAP 4.11, we just clone the very latest version of IO. But that now requires GAP 4.12. I could try to fix the CI test setup to resolve that, but it doesn't seem worth the effort, instead I would suggest to just not bother with testing against GAP 4.11 anymore. That said, if you'd prefer to keep that, I can look into a fix/workaround for the issue (likely by explicitly installing exactly IO 4.8.0 (nothing newer) when using GAP 4.11

[fingolfin]

@gaehler ping

[fingolfin]

@gaehler thank you for approving. How would you like to handle merging and releasing? Should I just merge this? Or are you waiting for something before doing that?

Regarding the CI test failures, I can either submit a PR to disable the tests against GAP 4.11, or I can try to fix them if you think this is important to have (personally I think most people on old GAP versions won't install this as an update, they are much more likely to get a new CaratInterface version as part of updating their entire GAP installation).

[gaehler]

@fingolfin I tried to merge the pull request (in the web interface), but nothing happens. If you can merge it, just go ahead. I'll make a release afterwards.