make it compatible with older make (debian stable - make 3.79.1)

Originally committed as revision 2377 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/Makefile

@@ -21,7 +21,13 @@ ifeq ($(CONFIG_RISKY),yes)
 OBJS+= asf.o
 endif
 
-ifeq ($(AMR_NB),yes) | ifeq ($(AMR_NB_FIXED),yes) | ifeq ($(AMR_WB),yes)
+ifeq ($(AMR_NB),yes)
+OBJS+= amr.o
+endif
+ifeq ($(AMR_NB_FIXED),yes)
+OBJS+= amr.o
+endif
+ifeq ($(AMR_WB),yes)
 OBJS+= amr.o
 endif