Building on MinGW fails due to missing "std::mutex"

[yaronct]

MinGW-w64 (GCC targetting Windows) that uses the "win32" threading model, doesn't provide some threading stuff like "std::mutex" (I'm not sue about the "posix" threading model), therefore building fails with:

./boost/math/special_functions/detail/bernoulli_details.hpp:563:9: error: ‘mutex’ in namespace ‘std’ does not name a type
563 | std::mutex m_mutex;
| ^~~~~

This can probably be solved by using threading stuff from the boost thread library (in the case of MinGW), as I think boost thread gives an API compatible with that of the standard C++ library ("std"). So I think, e.g. using "boost::mutex" should be compatible with "std::mutex". I haven't tried it though. Alternately, building boost math can be disabled on MinGW (possibly only for the "win32" threading model).

Tested against boost 1.78.0.

[mborland]

Can you try the linked PR, and let me know if it solves your issue?

[yaronct]

Thanks, @jzmaddock, #753 solves the problem. Or at least eliminates it 😄