Consistently linking all libraries with SYSTEM attribute

[marcelwa]

This PR builds upon #437 by consistently linking all libraries as SYSTEM because compiler warnings generated in third-party code are still propagated to including projects.

[codecov-commenter]

Codecov Report

Merging #469 (fadd7b9) into master (75380ce) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #469   +/-   ##
=======================================
  Coverage   83.61%   83.61%           
=======================================
  Files         134      134           
  Lines       15230    15230           
=======================================
  Hits        12734    12734           
  Misses       2496     2496           

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[hriener]

Thanks!