Allow selecting only the vp6f decoder.

This fixes the following option set:
  --disable-encoders --disable-decoders --disable-parsers --disable-bsfs
  --disable-muxers --disable-demuxers --disable-protocols --enable-decoder=vp6f

Originally committed as revision 9640 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -192,6 +192,7 @@ OBJS-$(CONFIG_VORBIS_ENCODER)          += vorbis_enc.o vorbis.o vorbis_data.o md
 OBJS-$(CONFIG_VP3_DECODER)             += vp3.o vp3dsp.o
 OBJS-$(CONFIG_VP5_DECODER)             += vp5.o vp56.o vp56data.o vp3dsp.o
 OBJS-$(CONFIG_VP6_DECODER)             += vp6.o vp56.o vp56data.o vp3dsp.o
+OBJS-$(CONFIG_VP6F_DECODER)            += vp6.o vp56.o vp56data.o vp3dsp.o
 OBJS-$(CONFIG_VQA_DECODER)             += vqavideo.o
 OBJS-$(CONFIG_WAVPACK_DECODER)         += wavpack.o
 OBJS-$(CONFIG_WMAV1_DECODER)           += wmadec.o wma.o mdct.o fft.o
@@ -354,6 +355,7 @@ OBJS-$(CONFIG_SNOW_DECODER)            += i386/snowdsp_mmx.o
 OBJS-$(CONFIG_VP3_DECODER)             += i386/vp3dsp_mmx.o i386/vp3dsp_sse2.o
 OBJS-$(CONFIG_VP5_DECODER)             += i386/vp3dsp_mmx.o i386/vp3dsp_sse2.o
 OBJS-$(CONFIG_VP6_DECODER)             += i386/vp3dsp_mmx.o i386/vp3dsp_sse2.o
+OBJS-$(CONFIG_VP6F_DECODER)            += i386/vp3dsp_mmx.o i386/vp3dsp_sse2.o
 endif
 
 ASM_OBJS-$(ARCH_ARMV4L)                += armv4l/jrevdct_arm.o     \