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

["howarth at bromo dot med.uc.edu" <gcc-bugzilla at gcc dot gnu dot org>]

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

--- Comment #13 from howarth at bromo dot med.uc.edu ---
Yes, I am pretty sure Apple has abandoned this for good. They only attempted
the change because it was assumed to be completely transparent as the compiler
looks in /usr/include by default. It wasn't appreciated that this would
cause...

radr://14320107 "/usr/bin/python reports back include directories that are
unpopulated"

where removal of /usr/include caused the system Python.h to point to an
unpopulated header directory in INCLUDEPY. While I provided them with a patch
to sysconfig.py to solve this, there is no stomach at Apple for pushing such
changes upstream as other packages like perl that hardcode compiler options
will need to be modified as well.
    While we could add this change to FSF gcc, I don't think we want to
encourage users not to install the system headers in /usr/include via the
Command Line Tools of Xcode as there are other configure scripts out there
which will also need to be patched to find /usr/include with `xcrun
--show-sdk-path`.
    If we made any change, I would rather it be a check in FSF gcc's configure
for the presence of /usr/include on darwin which provided the appropriate error
message to the user that the Command Line Tools needs to be installed.