CMake builds: support system-installed CaDiCaL

[tautschnig]

Just like we support system-installed MiniSat2 we now support CaDiCaL when selecting "system-cadical" as solver.

Fixes: #8113

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fd28cb2) 79.09% compared to head (43365c9) 79.69%.
Report is 2 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8159      +/-   ##
===========================================
+ Coverage    79.09%   79.69%   +0.59%     
===========================================
  Files         1695     1680      -15     
  Lines       196589   195104    -1485     
===========================================
- Hits        155493   155479      -14     
+ Misses       41096    39625    -1471     

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[peterschrammel]

Does this concern CMake only?

[tautschnig]

Does this concern CMake only?

In the make-based build system the user has to manually run the cadical-download target, or could choose to skip this any instead set variables appropriately.