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

[gdesmott]

No description provided.

[codecov]

Codecov Report

Merging #26 (3defd65) into master (529e86f) will decrease coverage by 0.12%.
The diff coverage is 91.30%.

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

Impacted Files	Coverage Δ
src/lib.rs	84.54% <83.33%> (-0.37%)	⬇️
src/test.rs	97.22% <100.00%> (+0.04%)	⬆️

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