Disable C++ models from being compiled without explicit request

[fmassa]

This has been added for testing C++ models in CI, but it shouldn't be part of the binaries.
A follow-up PR / commit will add proper CI for testing the models, which will require making it compile for OSX / Windows first

[codecov-io]

Codecov Report

Merging #1535 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1535   +/-   ##
======================================
  Coverage    64.6%   64.6%           
======================================
  Files          87      87           
  Lines        6742    6742           
  Branches     1034    1034           
======================================
  Hits         4356    4356           
  Misses       2085    2085           
  Partials      301     301

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