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 <jow@openwrt.org>

multimedia/motion/Makefile

@@ -1,5 +1,5 @@
 #
-# Copyright (C) 2008-2011 OpenWrt.org
+# Copyright (C) 2008-2015 OpenWrt.org
 #
 # This is free software, licensed under the GNU General Public License v2.
 # See /LICENSE for more information.
@@ -9,7 +9,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=motion
 PKG_VERSION=3.4.0-20141018-$(PKG_SOURCE_VERSION)
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_MAINTAINER:=Roger D <rogerdammit@gmail.com>
 PKG_LICENSE:=GPLv2