Fixed iOS build (#35)
Fixed iOS build (#35)

When `500 >= _XOPEN_SOURCE <= 599` vsnprintf and a few other standard C functions will give a compiler error like this: `implicit declaration library function 'vsnprintf' with type`. Bumping this number to 600 resolved errors for iOS: Apple Clang builds.
AndreRenaud/PDFGen,AndreRenaud/PDFGen,AndreRenaud/PDFGen