[Bug target/87243] FSF needs to use xcrun on darwin18 to find system headers in SDK

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87243

--- Comment #1 from Iain Sandoe <iains at gcc dot gnu.org> ---
I am strongly against making GCC's configure depend on xcrun.  It is quite
possible that GCC could be used, for example, with PureDarwin - or on systems
without any Xcode install (using headers built from the OpenSource packages).

So, yes we have to find a solution to finding the headers,
but no, (at least I don't agree) that trying into yet another tool we don't
control is that solution.