Added -DPNG_CONFIGURE_LIBPNG to the makefile CFLAGS

to prevent warnings about direct access to png structs by libpng functions
while building libpng.  They need to be tested, especially
those using compilers other than gcc.
diff --git a/scripts/makefile.openbsd b/scripts/makefile.openbsd
index 1d21fd8..736fb6d 100644
--- a/scripts/makefile.openbsd
+++ b/scripts/makefile.openbsd
@@ -20,7 +20,7 @@
 
 HDRS=	png.h pngconf.h
 
-CFLAGS+= -W -Wall
+CFLAGS+= -DPNG_CONFIGURE_LIBPNG -W -Wall
 CPPFLAGS+= -I${.CURDIR}
 
 NOPROFILE= Yes