* as.c (use_gnu_debug_info_extensions) : New variable.

* as.c (use_gnu_debug_info_extensions) : New variable.
 (parse_args) : Accept new --gstabs+ option, and set `use_gnu_debug_info_extensions'.
 (show_usage) : Document --gstabs+ option.
* as.h (use_gnu_debug_info_extensions) : New extern declaration.
* stabs.c (stabs_generate_asm_file) : If `use_gnu_debug_info_extensions' is set
  add the compilation directory to the stabs debug info.
* doc/as.texinfo : Document --gstabs+ option.
* NEWS: Mention new feature.