Fix build when linking against musl

[Calinou]

musl doesn't define PTHREAD_MUTEX_RECURSIVE, so it has to be done manually.

From my testing, this seems to work when using both GCC + glibc and GCC + musl (on Fedora 31 and Alpine respectively).

[akien-mga]

musl doesn't define this macro, so it has to be done manually. Since there doesn't seem to be a macro to detect whether musl is used, we should always define the macro on Linux.

I think you can use !defined(__GLIBC__): https://stackoverflow.com/questions/4266354/how-to-tell-if-glibc-is-used
It might still let through some non-GNU C libraries which define __GLIBC__ for compatibility, but at least musl doesn't seem to: http://git.musl-libc.org/cgit/musl/tree/include/features.h

[akien-mga]

Any chance to see this reviewed/merged? We use it downstream in Godot, would be good to have it upstream for others.

[Calinou]

Bump (again) 🙂

cc @kmammou