Also, while POSIX is a nice base, it stops you using Windows, and also almost every non-trivial program ends up with a bunch of autoconf (which has to be updated every so often) to handle differences between linux/BSD/Mac.

I agree the lack of compliant POSIX on Windows is annoying. However, unless you rely on 3rd-party libraries, you can use #ifdef to write OS-specific code without autoconf.