So, as the title says, build.xml is broken on Windows 8.
It is a very easy-to-fix problem, i just changed "Windows 7" to "Windows 8 " in the file, but i'm not good with git so i can't do a proper Pull Request.
Hope it gets fixed soon :)

Using osfamily="windows" instead of os="Windows 7" seems to work.
It only works from Ant 1.7 onwards.

Source: https://github.com/SirSengir/BuildCraft/blob/master/build.xml#L163; https://ant.apache.org/manual/Tasks/exec.html#background

Do you have an error log? Also, Github has some nice tutorials at help.github.com ;)
"Windows" itself might work without the actual version number behind it (Vista/7/8)

I'll be re-working the build.xml when I merge the client and common package together. I'll make sure to add a Win8 option

Should be fixed in latest commit.