cmake: fix GSL linking.

[KaiSzuttor]

fixes build with gsl.

[codecov]

Codecov Report

Merging #2429 into python will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           python   #2429    +/-   ##
=======================================
+ Coverage      72%     72%   +<1%     
=======================================
  Files         393     393            
  Lines       18483   18483            
=======================================
+ Hits        13411   13434    +23     
+ Misses       5072    5049    -23

Impacted Files	Coverage Δ
src/core/layered.cpp	78% <0%> (ø)	⬆️
src/core/domain_decomposition.cpp	95% <0%> (+4%)	⬆️
src/core/lattice.cpp	89% <0%> (+6%)	⬆️

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[KaiSzuttor]

@RudolfWeeber a similar fix should be applied to the 4.0.1 release