[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.vcwin32 b/scripts/makefile.vcwin32
index 4b9c7d7..b17f6a4 100644
--- a/scripts/makefile.vcwin32
+++ b/scripts/makefile.vcwin32
@@ -22,6 +22,10 @@
 CP = copy
 RM = del
 
+# Pre-built configuration
+# See scripts/pnglibconf.mak for more options
+PNGLIBCONF_H_PREBUILT = scripts/pnglibconf.h.prebuilt
+
 # File extensions
 O=.obj
 
@@ -37,8 +41,7 @@
 # Targets
 all: libpng.lib
 
-# see scripts/pnglibconf.mak for more options
-pnglibconf.h: scripts\pnglibconf.h.prebuilt
+pnglibconf.h: $(PNGLIBCONF_H_PREBUILT)
        $(CP) $< $@
 
 png$(O): png.h pngconf.h pnglibconf.h pngpriv.h pngstruct.h pnginfo.h pngdebug.h