Fix amr_nb-fixed build.

Originally committed as revision 5324 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/Makefile

@@ -245,8 +245,8 @@ endif
 
 ifeq ($(AMR_NB_FIXED),yes)
 AMROBJS= amr.o
-AMREXTRALIBS+= amr/*.o
-AMRLIBS=amrlibs
+EXTRAOBJS += amr/*.o
+EXTRADEPS=amrlibs
 CLEANAMR=cleanamr
 endif
 
@@ -423,12 +423,8 @@ LIBMAJOR=$(LAVCMAJOR)
 endif
 TESTS= imgresample-test dct-test motion-test fft-test
 
-EXTRAOBJS = $(AMREXTRALIBS)
-
 include $(SRC_PATH)/common.mak
 
-$(LIB): $(AMRLIBS)
-
 amrlibs:
 	$(MAKE) -C amr spclib fipoplib