[mcbarton]

A user may not want to build and install all of xeus-cpps many kernels (or may have their own custom kernels they want to try out). This PR introduces an option so that a user can choose at configure time what kernels to build.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.90%. Comparing base (65584a8) to head (bfba333).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #506   +/-   ##
=======================================
  Coverage   72.90%   72.90%           
=======================================
  Files          18       18           
  Lines        1203     1203           
  Branches      113      113           
=======================================
  Hits          877      877           
  Misses        326      326           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[JohanMabille]

I'm not sure to get the benefit of this change: there is a single xeus-cpp binary built, the distinction between kernels is done in the many kernelspec files which are small json files. Besides, the parametrized kernelspec which has been approved and is about to be implemented in the Jupyter ecosystem will remove the need for many kernelspec files.