[Bug target/57792] toplevel configure should enable "--with-sysroot="`xcrun --show-sdk-path`"" for darwin13 and later

[mrs at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57792

--- Comment #15 from mrs at gcc dot gnu.org <mrs at gcc dot gnu.org> ---
Mandating commands line tools is fine.  Would be nice if everything worked
flawlessly if no optional package had to be installed, but I'm pragmatic.