compile xvmc only when requested

Originally committed as revision 2088 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -14,7 +14,7 @@ OBJS= common.o utils.o mem.o allcodecs.o \
       mpegvideo.o jrevdct.o jfdctfst.o jfdctint.o\
       mpegaudio.o ac3enc.o mjpeg.o resample.o dsputil.o \
       motion_est.o imgconvert.o imgresample.o \
-      mpeg12.o xvmcvideo.o mpegaudiodec.o pcm.o simple_idct.o \
+      mpeg12.o mpegaudiodec.o pcm.o simple_idct.o \
       ratecontrol.o adpcm.o eval.o dv.o error_resilience.o \
       fft.o mdct.o mace.o huffyuv.o cyuv.o opts.o raw.o h264.o golomb.o \
       vp3.o asv1.o 4xm.o cabac.o ffv1.o ra144.o ra288.o vcr1.o
@@ -38,6 +38,10 @@ ifeq ($(CONFIG_RISKY),yes)
 OBJS+= h263.o msmpeg4.o h263dec.o svq1.o rv10.o wmadec.o indeo3.o
 endif
 
+ifeq ($(HAVE_XVMC_ACCEL),yes)
+OBJS+= xvmcvideo.o
+endif
+
 # currently using liba52 for ac3 decoding
 ifeq ($(CONFIG_AC3),yes)
 OBJS+= a52dec.o