Now that I merged this:

static strings cause exit time constructors. Maybe this could be a const char[] instead? (not sure if Evan told you to change this :-P)

More interestingly, the prefix is probably a UCS2 string, is that right? Do we do the right thing with 0 bytes? Or should we utf8->utf16 convert this? Since this is for international cl.exe versions, the manual should probably say something about the encoding here too.