Add EXTRADEPS variable for extra dependencies that need to be built first.

Originally committed as revision 5323 to svn://svn.ffmpeg.org/ffmpeg/trunk

common.mak

@@ -24,7 +24,7 @@ OBJS := $(OBJS) $(ASM_OBJS) $(CPPOBJS)
 STATIC_OBJS := $(OBJS) $(STATIC_OBJS)
 SHARED_OBJS := $(OBJS) $(SHARED_OBJS)
 
-all: $(LIB) $(SLIBNAME)
+all: $(EXTRADEPS) $(LIB) $(SLIBNAME)
 
 $(LIB): $(STATIC_OBJS)
 	rm -f $@