fix generation of auto-compiled file names on mingw systems

fix generation of auto-compiled file names on mingw systems

* libguile/load.c (canonical_to_suffix, scm_primitive_load_path):
* module/ice-9/boot-9.scm (load-in-vicinity):
* module/system/base/compile.scm (compiled-file-name): If the canonical
  path of a file is a DOS-style path with a drive letter, turn it into a
  path suffix it by removing the colon and prefixing a "/".

Inspired by a patch from Jan Nieuwenhuizen.