[Bug bootstrap/82091] [5 only] Build failure on macOS 10.13 with Xcode 9

[fxcoudert at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82091

--- Comment #2 from Francois-Xavier Coudert <fxcoudert at gcc dot gnu.org> ---
Yes, on 6 and 7 the headers are included early, as my patch is doing here.