Since ea7b8528 (#26422), autogen.sh overwrites the
build-aux/config.{guess, sub} files (installed there by autoreconf)
with the depends/config.{guess, sub} files if these are newer.
The autoreconf tool copies them from it's share/autoconf/build-aux/
directory. Specifically on NixOS, the share/autoconf/build-aux/
files are located in the nix-store and are read-only. autoreconf
preserves the read-only permissions when copying. Overwriting them
with our depends/config.{guess, sub} subsequently fails.
To make sure we can overwrite the files, we set write permissions to
the current user and group before overwriting. This fixes the problem
on NixOS.
fixes#27873: Can't copy to 'build-aux/config.guess' in autoconf.sh: Permission denied