The MMAL_CFLAGS / MMAL_LIBS will be used in the Makefile.am instead of
the plugin defined flags, since there are multiple plugins, and in
addition of a HAVE_MMAL conditional.

In addition, -L flags are LIBADD flags, and not LDFLAGS flags, so it's
actually put into MMAL_LIBS instead of defining a MMAL_LDFLAGS.