Linux installer: exclude backends from library compatibility test

Engine backends are loaded dynamically, and Ardour can start without
them. This is manly to relax the requirement of having libpulse.so.

libardour itself depends on libasound, so the ALSA backend is always
present, and the jack-backend dynamically dlopen()s libjack.

tools/linux_packaging/stage2.run.in

@@ -498,7 +498,7 @@ then
 fi
 	
 # check the libs
-LIB_FILES=$(find ${BUNDLE_DIR}/lib -name "*.so" -type f)
+LIB_FILES=$(find ${BUNDLE_DIR}/lib -name "*.so" -type f | grep -v backends)
 
 for path in $LIB_FILES
 do