Move fix for clang bug to C file.

[jeromekelleher]

This moves the workaround for the clang bug (#721) from the header file to a C file, to address very good points made by @bhaller. The fix is now local to C code, so we don't need to check for C++ any more, and library users will no longer have this workaround foisted on them unwittingly.

@molpopgen, do you see any problems with doing this?

[bhaller]

@petrelharp I guess this should be pulled into SLiM once this PR is approved, so I'm tagging you to put it on your radar. :->

[jeromekelleher]

Ended up having to take a different approach here because we used isnan etc in the test suite as well. Same basic outcome though.

Thanks for taking a look @bhaller.

The 0.99.4 version should be tagged in the next day or so, so hopefully we can sync up on that for SLiM.

[codecov]

Codecov Report

Merging #758 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #758   +/-   ##
=======================================
  Coverage   87.69%   87.69%           
=======================================
  Files          24       24           
  Lines       19388    19388           
  Branches     3640     3640           
=======================================
  Hits        17002    17002           
  Misses       1297     1297           
  Partials     1089     1089           

Flag	Coverage Δ
#c_tests	85.28% <ø> (ø)
#python_c_tests	90.16% <ø> (ø)
#python_tests	99.02% <ø> (ø)

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Impacted Files	Coverage Δ
c/tskit/core.h	100.00% <ø> (ø)
c/tskit/tables.c	81.17% <ø> (ø)

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[molpopgen]

This should be fine.

[jeromekelleher]

@benjeffery, this look good to you?

[benjeffery]

LGTM