cmdutils: Only do the windows-specific commandline parsing on _WIN32

Fixes commandline parsing on Cygwin (on 64 bit, and on very recent
32 bit), where the configure check does find the CommandLineToArgvW
function (since it exists in the link libraries and in the headers),
but whose GetCommandLineW() only returns the application's path.
(This is due to a cygwin internal optimization, see
http://cygwin.com/ml/cygwin/2013-07/msg00538.html for details.)
Arguments are only given through main's argc/argv, and they're already
UTF-8.

Signed-off-by: Martin Storsjö <martin@martin.st>
