🩹 make GMP dependency opt-in instead of used-if-found

[burgholzer]

Description

This PR changes the CMake configuration so that GMP, which can be used in the ZX library, needs to be explicitly added (opt-in) instead of automatically being used (actually unconditionally, without an opt-out option).

This has popped up in cda-tum/mqt-qcec#396 since delocate fails to properly repair the produced wheel. This is a result of gmp being automatically installed on macOS runners via brew. However, this gets the latest system version, which is not compatible with the minimum macOS targets that we set. I believe this only popped up now, because there has been a delocate update that catches this.
In lack of a better option at the moment, this PR makes GMP support entirely opt-in, which disables it for wheel builds and resolves the underlying issue.
Anyone that wants to build MQT Core with GMP support, can now configure CMake with -DMQT_CORE_WITH_GMP

In the future, it might be worth investigating whether there is a better way to do this, e.g., by manually building gmp with the required compatibility.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.6%. Comparing base (acb8895) to head (315ee62).
Report is 110 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #608   +/-   ##
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  Coverage   91.6%   91.6%           
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  Files        147     147           
  Lines      14635   14635           
  Branches    2341    2341           
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+ Hits       13408   13409    +1     
+ Misses      1227    1226    -1     

Flag	Coverage Δ
cpp	91.3% <ø> (+<0.1%)	⬆️
python	99.7% <ø> (ø)

see 1 file with indirect coverage changes