I think you need to save the output of reascript_vararg.php to reascript_vararg.h to get a valid C++ code you can build. I haven't tried though...
It's part of their build scripts:
Windows:
https://github.com/Jeff0S/sws/blob/2...n.vcxproj#L104
OS X:
https://github.com/Jeff0S/sws/blob/2...ld.command#L16