compil against external libbluray

Hi
Regarding all the activity against libbluray/libaacs and libbdplus it is pretty interesting to be able to compil xbmc against a self compiled libbluray installed in a custom path.

For instance after compiling libbluray,  installing it in /home/sasha/source_code/build/xbmc  and defining those required environment variables
export PKG_CONFIG_PATH=/home/sasha/source_code/build/xbmc/lib/pkgconfig
export LD_LIBRARY_PATH=/home/sasha/source_code/build/xbmc/lib
, the  configure script fail with this error message : 
configure: error: Unable to determine soname of libbluray library

This patch allow to proceed without failing and compiling xbmc against custom libbluray.