fix makefile to read variables from environment and overrides

[jeevb]

No description provided.

[codecov]

Codecov Report

Merging #104 (64316a3) into master (25c0a10) will increase coverage by 1.16%.
The diff coverage is n/a.

❗ Current head 64316a3 differs from pull request most recent head 8ac31a8. Consider uploading reports for the commit 8ac31a8 to get more accurate results

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   68.50%   69.67%   +1.16%     
==========================================
  Files          31       31              
  Lines        1667     1385     -282     
==========================================
- Hits         1142      965     -177     
+ Misses        447      342     -105     
  Partials       78       78              

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see 28 files with indirect coverage changes

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