Fix for portability issue with GNU make extension 'sort'
Fix for portability issue with GNU make extension 'sort'

upnp/sample/Makefile.am:67: warning: sort \
upnp/sample/Makefile.am:67:                     $(tv_ctrlpt_SOURCES: non-POSIX variable name
upnp/sample/Makefile.am:67: (probably a GNU make extension)

Reference:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13771#8
(cherry picked from commit dcbbc30f5c41746a8c8b671dade799760b432fb1)

mrjimenez/pupnp,BelledonneCommunications/libupnp,mrjimenez/pupnp,commshare/pupnp,commshare/pupnp,BelledonneCommunications/libupnp,BelledonneCommunications/libupnp,commshare/pupnp,BelledonneCommunications/libupnp,mrjimenez/pupnp,mrjimenez/pupnp