Make boost::math::ccmath::fminf and boost::math::ccmath::fminl constexpr

[quxflux]

The publicly visible declarations of boost::math::ccmath::fminf and boost::math::ccmath::fminfl are neither marked inline nor constexpr, which causes "multiple definition" linker errors when the boost/math/ccmath/fmin.hpp header gets included in multiple translation units of a binary.

Therefore this PR marks said functions constexpr.

Fixes #1145

[mborland]

LGTM. Approved the CI run. Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.76%. Comparing base (70cdb37) to head (dff806a).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1146      +/-   ##
===========================================
+ Coverage    93.71%   93.76%   +0.05%     
===========================================
  Files          774      772       -2     
  Lines        61271    61251      -20     
===========================================
+ Hits         57422    57434      +12     
+ Misses        3849     3817      -32     

see 3 files with indirect coverage changes

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