[Bug other/79885] fix-includes does not honor the value passed to --with-build-sysroot=, looks for /usr/include instead of looking within the SDK

[jeremyhu at macports dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79885

--- Comment #1 from Jeremy Huddleston Sequoia <jeremyhu at macports dot org> ---
And note that I'm not using --with-sysroot because I don't want this to affect
the compiler installed by 'make install', just what is used to build gcc
itself.