Fix build on musl

[NTULINUX]

While probably not perfect; this patch series allows LinuxCNC to be successfully compiled on a musl system.

[NTULINUX]

Please see #3185 for discussion on the changes.

[BsAtHome]

The call to pthread_setaffinity_np() is wrong. It is performed before the thread is created by pthread_create(). The thread's ID is unknown at that time. In my earlier description, I wrote that such change should go after creating the thread (see #3185 (comment)).

Also, using pthread_setaffinity_np() must be conditional and only be used if pthread_attr_setaffinity_np() is not available. Therefore, you need to have a autoconf check.

[NTULINUX]

I never wrote an autoconf test before for a C function but I'll try :) I have some experience with autotools and I think there's a few places I know to look for example code.