Use pkgconfig to locate ALSA libs (#6119)

* This allows building when ALSA libs are in a non-standard location. PKG_CONFIG_PATH alone is not enough as the final link fails. Adding this makes the final link succeed.

* The extra LIBS flag for alsa is not needed so removing.

platform/x11/detect.py

@@ -155,7 +155,7 @@ def configure(env):
 	if os.system("pkg-config --exists alsa")==0:
 		print("Enabling ALSA")
 		env.Append(CPPFLAGS=["-DALSA_ENABLED"])
-		env.Append(LIBS=['asound'])
+		env.ParseConfig('pkg-config alsa --cflags --libs')
 	else:
 		print("ALSA libraries not found, disabling driver")