Add API to preserve libs flags from system libs (PKG_CONFIG_ALLOW_SYSTEM_LIBS)

[gdesmott]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 91.30435% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.28%. Comparing base (529e86f) to head (3defd65).
⚠️ Report is 118 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lib.rs	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
- Coverage   90.41%   90.28%   -0.13%     
==========================================
  Files           2        2              
  Lines        1419     1431      +12     
==========================================
+ Hits         1283     1292       +9     
- Misses        136      139       +3     

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