build.jl: refactor how we invoke configure

[fingolfin]

With the new variant, it's trivial to keep the debug and non-debug settings in
sync; also, if people add or remove configure options, that is now easy to see
in a diff.

Motivated by reviewing PR #104 by @sebasguts ;-)

[wbhart]

Thanks for this. I'm reluctant to merge it just yet until our tests are passing again though. I will wait until then and come back to this.

[fingolfin]

Of course, that makes perfect sense, I wouldn't expect anything else (I was in fact kinda relying on the CI tests to fully verify what I did here)

[codecov]

Codecov Report

Merging #109 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #109   +/-   ##
=====================================
  Coverage       0%     0%           
=====================================
  Files          30     30           
  Lines        2588   2588           
=====================================
  Misses       2588   2588

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