build mpegaudio.c and ac3enc.c only if needed

Originally committed as revision 5648 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -10,7 +10,7 @@ CFLAGS=$(OPTFLAGS) -DHAVE_AV_CONFIG_H -I.. -I$(SRC_PATH)/libavutil \
 
 OBJS= bitstream.o utils.o mem.o allcodecs.o \
       mpegvideo.o jrevdct.o jfdctfst.o jfdctint.o\
-      mpegaudio.o ac3enc.o mjpeg.o resample.o resample2.o dsputil.o \
+      mjpeg.o resample.o resample2.o dsputil.o \
       motion_est.o imgconvert.o imgresample.o \
       mpeg12.o mpegaudiodec.o pcm.o simple_idct.o \
       ratecontrol.o adpcm.o eval.o error_resilience.o \
@@ -26,6 +26,9 @@ HEADERS = avcodec.h
 ifeq ($(CONFIG_AASC_DECODER),yes)
     OBJS+= aasc.o
 endif
+ifeq ($(CONFIG_AC3_ENCODER),yes)
+    OBJS+= ac3enc.o
+endif
 ifeq ($(CONFIG_ALAC_DECODER),yes)
     OBJS+= alac.o
 endif
@@ -119,6 +122,9 @@ endif
 ifneq ($(CONFIG_MACE3_DECODER)$(CONFIG_MACE6_DECODER),)
     OBJS+= mace.o
 endif
+ifeq ($(CONFIG_MP2_ENCODER),yes)
+    OBJS+= mpegaudio.o
+endif
 ifeq ($(CONFIG_MSRLE_DECODER),yes)
     OBJS+= msrle.o
 endif