[Bug other/70268] add option -ffile-prefix-map to map one directory name (old) to another (new) in __FILE__, __BASE_FILE__and __builtin_FILE()

[boris at kolpackov dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70268

--- Comment #11 from Boris Kolpackov <boris at kolpackov dot net> ---
Third revision of the patch:

https://gcc.gnu.org/ml/gcc-patches/2017-12/msg00544.html