[feat] Use the variable built-in machinery in the build system classes

[jjotero]

Closes #2030.

[codecov-commenter]

Codecov Report

Merging #2095 (7012ac3) into master (f55aa1c) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2095      +/-   ##
==========================================
- Coverage   86.30%   86.23%   -0.07%     
==========================================
  Files          53       53              
  Lines        9352     9305      -47     
==========================================
- Hits         8071     8024      -47     
  Misses       1281     1281              

Impacted Files	Coverage Δ
reframe/core/buildsystems.py	96.10% <100.00%> (-0.24%)	⬇️
reframe/utility/typecheck.py	94.54% <0.00%> (-0.91%)	⬇️

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[vkarak]

lgtm