[Bug libstdc++/81797] gcc 7.1.0 fails to build on macOS 10.13 (High Sierra):

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81797

--- Comment #33 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Marc Glisse from comment #28)
> Generally, I don't understand why we are linking sources in the build
> directory instead of passing -I flags pointing directly to the source
> directory.

I think it's because the directory structures and relative layout of files are
different. The build dir contains symlinks to assemble the right set of files
from various different dirs, like include/std and config/locale etc. into a
single location.