Use system zopfli library and respect CFLAGS.

--- apngopt-1.4-src/Makefile
+++ apngopt-1.4-src/Makefile
@@ -1,10 +1,10 @@
 PACKAGE    = apngopt
 CC         = gcc
-SRC_DIRS   = . 7z zopfli
+SRC_DIRS   = . 7z
 CFLAGS     = -Wall -pedantic
-CFLAGS_OPT = -O2
+#CFLAGS_OPT = -O2
 CFLAGS_7Z  = -Wno-sign-compare -Wno-reorder -Wno-maybe-uninitialized -Wno-parentheses
-LIBS       = -lstdc++ -lm -lpng -lz
+LIBS       = -lstdc++ -lm -lpng -lz -lzopfli
 
 INCUDE_DIRS := $(addprefix -I./, $(SRC_DIRS))
 OBJ_DIRS := $(addprefix obj/, $(SRC_DIRS))
@@ -16,7 +16,7 @@
 all : $(PACKAGE)
 
 $(PACKAGE) : objdirs $(OBJECTS)
-	$(CC) -o $@ $(OBJECTS) -s $(LIBS)
+	$(CC) -o $@ $(OBJECTS) $(LIBS)
 
 objdirs :
 	mkdir -p $(OBJ_DIRS)