Define pthread mutex as recursive

[CasualPokePlayer]

For consistent semantics against other rc_mutex_t implementations (mostly more making sense with my other PRs; regardless, the current Windows mutex implementation already has recursive semantics).

[Jamiras]

I don't think the code is currently reliant on recursive locks. There's a few places with explicit comments indicating a lock is assumed to be held prior to calling the function (i.e.

rcheevos/src/rc_client.c

Line 5005 in 32917bd

/* ASSERT: this should only be called if the mutex is held */

) to eliminate the need for recursive locks.

Having said that, the changes seem reasonable, and do provide a better level of compatibility in case recursive locks end up getting introduced.

[CasualPokePlayer]

Note probably want to introduce an !defined(_3DS) for the _XOPEN_SOURCE define guard when #377 is merged.