Fix CMake install prefix when not set at configure time

[cschreib]

Closes #196

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.16%. Comparing base (675885f) to head (25763e6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #197   +/-   ##
=======================================
  Coverage   94.16%   94.16%           
=======================================
  Files          30       30           
  Lines        1747     1747           
=======================================
  Hits         1645     1645           
  Misses        102      102           

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