[Bug target/108287] AVR build: gcc/config/avr/t-avr tries to edit the source tree

["gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108287

--- Comment #4 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
Well, updating or creating some auto-generated files is intentional.

What's not supported as of GCC documentation is configure'ing in the source
tree:

https://gcc.gnu.org/install/configure.html

> First, we **highly** recommend that GCC be built into a separate directory
> from the sources which does not reside within the source tree.
> This is how we generally build GCC; building where srcdir == objdir
> should still work, but doesn’t get extensive testing; building where
> objdir is a subdirectory of srcdir is unsupported.

The reason why it does not work for you might be:

1) Maybe you changed avr-mcus.def to support more devices. This change will
trigger more changes, for example to auto-generated documentation (texi) bits. 
This means you are basically a maintainer, which in turn means you migth have
more jobs to do, or tools to use than a simple user who just builds GCC from
source.

2) When you get the sources from some repo like git, the checked-out sources
might have timestamps that don't reflect their true state. This triggers make
to re-build auto-generated files, even though no prerequisite was changed and
the targets need not be rebuilt.

To fix this, run
  ./contrib/gcc_update --touch
from the top-level source dir.  This script will touch some source files and
fix their timestamps.  You obviously need write permission for that.