WIP: [ci] Avoid build failure for OSX + clang

[bchretien]

Cf. #40.

[codecov]

Codecov Report

Merging #41 into master will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage      87%   86.94%   -0.06%     
==========================================
  Files          52       52              
  Lines        3570     3570              
  Branches      256      256              
==========================================
- Hits         3106     3104       -2     
- Misses        320      322       +2     
  Partials      144      144

Impacted Files	Coverage Δ
tests/polynomial.cc	96.31% <0%> (-0.57%)	⬇️

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[coveralls]

Coverage remained the same at 85.008% when pulling ee4d701 on bchretien:fix/ci-osx-ipopt into e13630d on roboptim:master.