__FILE__ gives unusable paths on NT

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=2733

Reported by Marcus Wellhardh wellhard@roxen.com

This:
  werror("__FILE__: %O\n", __FILE__);
Gives:
  __FILE__: "/${PIKE_MODULE_PATH}/IntraWise.pmod/LicenseVariable.pike"