Adapt MemoryOrder definition for C++ 20

[SeverinLeonhardt]

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When compiling the code with C++ 20 the following error message is produced:

enumerator value for ‘MEMORY_ORDER_RELAXED’ must have integral or unscoped enumeration type

This is because with C++ 20 the type of those values changed to an enum class. The static_cast extracts the numeric values so they can be used here as before.

[absurdfarce]

Thanks for the contributions @SeverinLeonhardt, this one as well as the other two PRs. Unfortunately I've been completely swamped recently (thus the delay in getting back to you) but I'll try to take a look at these PRs next week!

[absurdfarce]

Finally got a chance to take a look at this one @SeverinLeonhardt. That's a nice and elegant way to solve that issue... and degrades nicely for the C++11 case (which is the default).

Many thanks for the solid contribution! And again, my apologies for the delay; we're trying to get back on track here.