Buildsystem: Allow environment C preprocessor flags.

[tlsa]

No description provided.

[codecov]

Codecov Report

Merging #169 (386d593) into main (bc5481d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #169   +/-   ##
=======================================
  Coverage   97.11%   97.11%           
=======================================
  Files           9        9           
  Lines        1596     1596           
  Branches      342      342           
=======================================
  Hits         1550     1550           
  Misses         24       24           
  Partials       22       22           

Flag	Coverage Δ
unittests	97.11% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bc5481d...386d593. Read the comment docs.