Split h264dsp and h264pred in configure.
Many H.264 derivatives, like RV40 and VP8, use the H.264 prediction functions
but not the weight/loopfilter functions.
This should reduce the size of builds with one of these derivatives but without
H.264 decoding itself.

Originally committed as revision 24741 to svn://svn.ffmpeg.org/ffmpeg/trunk
