If you're just looking to do open source for the fun of writing software then by all means ignore portability(which is a totally legit approach).

However if you're goal is to have an impact with your software then supporting win32 is a good bet since you're going to find an order of magnitude more users on win32. You'll also probably want iOS, Android and other platforms that aren't drop-in ports for POSIX if you want to provide a reasonable interface.