[libpng16] Allow an easy replacement of the default pre-built configuration

header with a custom header, via the make PNGLIBCONF_H_PREBUILT
macro.
diff --git a/scripts/makefile.openbsd b/scripts/makefile.openbsd
index 5599d75..a1dccc8 100644
--- a/scripts/makefile.openbsd
+++ b/scripts/makefile.openbsd
@@ -31,11 +31,14 @@
 DOCS=	ANNOUNCE CHANGES INSTALL KNOWNBUG LICENSE README TODO Y2KINFO \
 	libpng-manual.txt
 
+# Pre-built configuration
+# See scripts/pnglibconf.mak for more options
+PNGLIBCONF_H_PREBUILT= scripts/pnglibconf.h.prebuilt
+
 .c.o:
 	$(CC) -c $(CPPFLAGS) $(CFLAGS) -o $@ $<
 
-# see scripts/pnglibconf.mak for more options
-pnglibconf.h: scripts/pnglibconf.h.prebuilt
+pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
 	cp $< $@
 
 pngtest.o:	pngtest.c