--- pngcheck-2.3.0/Makefile.unx.cflags	2017-09-18 01:52:37.482685393 +0200
+++ pngcheck-2.3.0/Makefile.unx	2017-09-18 01:53:54.181114655 +0200
@@ -23,7 +23,7 @@
 CC = gcc
 LD = gcc
 RM = rm
-CFLAGS = -O -Wall -DUSE_ZLIB
+CFLAGS += -O -Wall -DUSE_ZLIB
 # [note that -Wall is a gcc-specific compilation flag ("all warnings on")]
 O = .o
 E =