[2.24 PATCH 1/3] Add utility macros for clang detection, and deprecation with messages.

[Paul Eggert]

Zack Weinberg wrote:
> It might still be a good idea as part of a complete revision of
> sys/cdefs.h + features.h, subsuming _all_ of the GCC version checks
> into this mechanism and rendering it easily extensible to more
> different compilers -- which is what Roland wanted -- but that is a
> separate project, not appropriate for 2.24 at this stage, and I do not
> want to mission-creep _this_ patchset even to the extent of
> introducing the skeleton of the mechanism.

Yes, thanks, that all sounds good.

For what it's worth, the Emacs source code is already implementing something 
similar to what Roland and I were proposing (using different macro names).