Cleaning mthread

inc/mthread/mthread.h

inc/mthread/mthread_internal.h

meson.build

src/mthread/mthread.c

src/mthread/mthread_cond.c

src/mthread/mthread_key.c

src/mthread/mthread_mutex.c

src/mthread/mthread_once.c

src/mthread/mthread_sem.c