sdl: overwrite libpth autodetection

The sdl libpth autodetection gets confused in case a host
version is present. Overwrite libpth autodection for the
target build dependent on BR2_PACKAGE_LIBPTHSEM_COMPAT and
use pth-config from staging directory.

Fixes [1]:

checking for pthreads... no
checking for pth-config... /usr/bin/pth-config
checking pth... yes
[...]
./src/SDL.c:33:17: fatal error: pth.h: No such file or directory
 #include <pth.h>

[1] http://autobuild.buildroot.net/results/733/73330e51b3c099d179004db0b9522ac13358b8f7



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>