Gnulib and nullptr

[Paul Eggert]

To support an approximation to C23's nullptr keyword I installed the 
attached patch into Gnulib. To test it a bit I'm using the new c-nullptr 
module in GNU diffutils, by preferring nullptr to NULL in the diffutils 
source. See:

https://git.savannah.gnu.org/cgit/diffutils.git/commit/?id=09f8e2b0a92f3d25325f5b1df20384140261ee10

Assuming this works out, at some point I would like to change Gnulib 
itself to prefer nullptr to NULL, due to nullptr's somewhat-better 
properties. There's no rush of course.

With current c-nullptr Gnulib .h files would continue to use NULL rather 
than nullptr, since we couldn't assume client code includes a <config.h> 
that defines nullptr on pre-C23 platforms, so perhaps at some point that 
could be improved as well.

0001-c-nullptr-new-module.patch
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