Make mktemp compatible with OSX.

OSX uses a slightly different mktemp from Linux. Linux uses GNU mktemp, while OSX uses the BSD version.
mktemp on OSX (and presumably BSD) returns an error if no arguments are supplied. Supplying some default name makes both GNU and BSD mktemp happy.

Tested on OSX 10.7, 10.8, Fedora 14 (Linux 2.6.35.14)