i encountered a Problem on OS X which is already discussed here: https://love2d.org/forums/viewtopic.php?f=5&t=9479&start=10
But i installed LUA 5.1 using MacPorts and here, it does work:
print(io.popen('uname -a','r'):read('*l')); Darwin haimimac.local 11.4.0 Darwin Kernel Version 11.4.0: Mon Apr 9 19:32:15 PDT 2012; root:xnu-1699.26.8~1/RELEASE_X86_64 x86_64
Did you forget to enable it for the Mac build?
Currently I have no way to enable my os detection on mac... maybe this is also a way of os detection ^^