[Bug other/91879] --with-build-sysroot doesn't work as expected

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91879

--- Comment #13 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Wed, 25 Sep 2019, stsp at users dot sourceforge.net wrote:

> Unfortunately I wasn't able to fully understand the
> idea you explain. You mention "sysroot" and "non-sysroot"
> builds, and you suggest to use --with-build-sysroot=
> together with --with-sysroot= (without providing an example
> of how to set --with-build-sysroot=, so I assume it should
> be equal to $DESTDIR). May I assume that in the "sysroot"

No, it should be $DESTDIR/$prefix/$target (and then you need a symlink 
usr -> . under that directory if GCC thinks /usr/include is the system 
include directory for that target).

> build all the -isystem pathes will be changed the way the
> sysroot prefix is replaced with build-sysroot prefix during
> libs build?

The relevant paths will be the ones in SYSROOT_CFLAGS_FOR_TARGET 
(--sysroot option, not -isystem).

> -build_tooldir=${tooldir}
> +build_tooldir=${build_sysroot_path}${tooldir}
> 
>  # Create a .gdbinit file which runs the one in srcdir
>  # and tells GDB to look there for source files.
> ------
> 
> What do you think guys? Does it break something?
> It definitely works for me.

That's not a correct change.  tooldir is a *host* path, not a target one, 
and the build sysroot can only correctly be prepended to *target* paths.