Fix order, fix variables that don't exist, like GST_LIBS_LIBS,
use $(LIBM) instead of -lm, and move _LIBS from LDFLAGS to LIBADD.
Spotted by Havard Graff.
If you're running things uninstalled, you might need to manually
remove the registry to get the plugin's features to show up
correctly. Also, you probably want to do a make clean to get rid
of the old plugin file.
