RFC: Improve the ability to override the cflags(), ldflags(), etc.

[sjkelly]

This improves the ability to override the cflags(), ldflags(), etc.
Used during the build process. One may want to override these flags
for example to add hardening options during compile time or linking.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.62%. Comparing base (d5a8618) to head (8767356).
Report is 20 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #920   +/-   ##
=======================================
  Coverage   84.62%   84.62%           
=======================================
  Files           3        3           
  Lines         826      826           
=======================================
  Hits          699      699           
  Misses        127      127           

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