[Bug bootstrap/57076] @ in the src directory name causes failure while building of gcc.info

[egallager at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57076

--- Comment #4 from Eric Gallager <egallager at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> Maybe the best thing would be reject this during configure.
>

Since I can't think of anything better, agreed.