[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.bor b/scripts/makefile.bor
index 70236d6..254de89 100644
--- a/scripts/makefile.bor
+++ b/scripts/makefile.bor
@@ -58,6 +58,10 @@
 # -M  generate map file
 LDFLAGS=-M -L$(ZLIB_DIR) $(MODEL_ARG) $(LDEBUG)
 
+# Pre-built configuration
+# See scripts/pnglibconf.mak for more options
+PNGLIBCONF_H_PREBUILT = scripts/pnglibconf.h.prebuilt
+
 ## Variables
 
 OBJS = \
@@ -111,8 +115,8 @@
 
 all: libpng pngtest
 
-# try !inlude scripts\pnglibconf.mak for more options
-pnglibconf.h: scripts\pnglibconf.h.prebuilt
+# try !include scripts\pnglibconf.mak for more options
+pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
 	$(CP) $< $@
 
 libpng: $(LIBNAME)