omit math.h under cpp

[petrelharp]

Thanks to @molpopgen, this seems to fix problems that prevent including the C files in C++ (see MesserLab/SLiM#101 (comment)). I have no idea if there's a better way to do this.

[jeromekelleher]

LGTM - great to get this sorted out before we tag C 0.99.4 too.

[codecov]

Codecov Report

Merging #755 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #755   +/-   ##
=======================================
  Coverage   87.69%   87.69%           
=======================================
  Files          24       24           
  Lines       19387    19387           
  Branches     3640     3640           
=======================================
  Hits        17001    17001           
  Misses       1297     1297           
  Partials     1089     1089           

Flag	Coverage Δ
#c_tests	85.27% <ø> (ø)
#python_c_tests	90.16% <ø> (ø)
#python_tests	99.02% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
c/tskit/core.h	100.00% <ø> (ø)

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[petrelharp]

Passing tests - shall we merge?

[jeromekelleher]

Yep - was just making sure that Windows was happy with this before giving the thumbs up.