motion: fix segmentation fault under musl libc

motion: fix segmentation fault under musl libc

During startup, motion calls pthread_getspecific() through motion_log()
before pthread_key_create() has been called yet. This works on glibc and
uclibc but segfaults on musl because motion is relying on undefined
behaviour here.

Move the pthread initialization before motion_startup() so that
tls_key_threadnr is initialized when motion_log() is called.

Also enforce the use of strerror_r() on musl by defining XSI_STRERROR_R
on all non-glibc systems because the supposed replacement code is broken
and crashes on musl.

References:
http://www.lavrsen.dk/foswiki/bin/view/Motion/BugReport2015x09x30x203633

Signed-off-by: Jo-Philipp Wich <redacted>