On GNU/kFreeBSD, the definition for alloca() can be found in the
system alloca.h

On GNU/kFreeBSD, sys/signal.h is only a wrapper around glibc signal.h
anyway, leading to a #include loop in this case.

On GNU/kFreeBSD, pthread_setname_np can be found in glibc's pthread.h
(same as on GNU/Linux).  pthread_np.h does not exist there.

Fix potential compilation error by ensuring __GLIBC__ is only evaluated
when actually defined.

When __GLIBC__ is defined, we do not need any additional headers on BSD
platforms (hence why using #elif).

Trying to evaluate __GLIBC__ will result in an error if is not defined,
if the preprocessor does not short-cut the evaluation.

Split the macros onto separate lines and define the result in a new
BX_USE_GLIBC_PTHREAD_SETNAME_NP macro to avoid duplication.