Check for CONFIG_LIBFOO_DECODER/CONFIG_LIBFOO_ENCODER instead of just

CONFIG_LIBFOO in the external libraries section.
This is more consistent with the rest of the Makefiles, it makes clearer what
is actually implemented and should be advantageous if we implement an external
library encoder where we previously just had the decoder and vice versa.

Originally committed as revision 19366 to svn://svn.ffmpeg.org/ffmpeg/trunk