Optimize compilation of cre.cpp

1. Remove unused "-lsdtc++" from the compilation stage as no linking is
done then.
2. Add our standard CFLAGS to the compilation of cre.cpp which brings in
-O3 -march=armv6 which enables optimizations.
It seemed strange that we compile a cpp file with gcc (as opposed to
g++), but I left it as is for now.

Makefile

@@ -124,7 +124,7 @@ djvu.o: %.o: %.c
 	$(CC) -c $(KPDFREADER_CFLAGS) -I$(DJVUDIR)/ $< -o $@
 
 cre.o: %.o: %.cpp
-	$(CC) -c -I$(CRENGINEDIR)/crengine/include/ -I$(LUADIR)/src $< -o $@ -lstdc++
+	$(CC) -c $(CFLAGS) -I$(CRENGINEDIR)/crengine/include/ -I$(LUADIR)/src $< -o $@
 
 lfs.o: $(LFSDIR)/src/lfs.c
 	$(CC) -c $(CFLAGS) -I$(LUADIR)/src -I$(LFSDIR)/src $(LFSDIR)/src/lfs.c -o $@