Include headers for optional libraries only when enabled.

Include headers for optional libraries only when enabled.

Include the headers for libavfilter, libswscale and libpostproc when
they are enabled (and thus linked in).

Without this change, Sun Studio will fail to link FFmpeg since two
inline functions defined in avfilter.h will have undefined references
to non-inline libavfilter functions.

Originally committed as revision 15665 to svn://svn.ffmpeg.org/ffmpeg/trunk