People whining about POSIX should just shut up. POSIX is by now a small part of the whole system. About the only thing you can write strictly within POSIX are mail daemons (well, maybe also ftp daemons). Almost everything else would need system-specific bits.
And that's OK, because otherwise there'd be no need for different operating system. We'd just all happily use One True Faceless OS.
And I also disagree about Windows being a horrible OS for build systems. It's actually a GREAT OS for build systems because it forces you to avoid hacky shortcuts like assuming that /usr/include always has all the relevant libraries or always installing binaries into /usr/bin.