Compilations warnings-as-errors when building from git

[Paul Eggert]

In <https://lists.gnu.org/r/coreutils/2022-01/msg00017.html> Assaf 
Gordon writes:

> I'm getting few warnings-as-errors when building the latest version from git 
> (using Debian 10 amd64 with gcc 8.3.0).

I wouldn't worry about these. We typically don't bother to pacify older 
GCC versions as they typically generate false alarms that are not worth 
worrying about.

In <https://lists.gnu.org/r/coreutils/2022-01/msg00018.html> Assaf 
Gordon writes:

> with clang-14 ( Debian clang version 
> 14.0.0-++20211220125923+c79a67196828-1~exp1~20211220130019.184 )

Those warnings can also be ignored (as we typically don't worry overmuch 
about bogus warnings from Clang either), except:

> warning: unknown warning option '-Wno-unsuffixed-float-constants' 
> [-Wunknown-warning-option]
>
>
> Which, I see was removed from gnulib in 2011,
> and reinstated just now in
> https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=0c8a563f65d44752b33aec42cceec25bd485f2d5

This appears to be a bug in that Gnulib commit. Presumably this code in 
m4/gnulib-common.m4:

      #if __GNUC__ + (__GNUC_MINOR__ >= 5) > 4 || (__clang_major__ + 
(__clang_minor__ >= 9) > 3)
      -Wno-unsuffixed-float-constants
      #endif

needs to be adjusted as far as Clang versions go. Do you happen to know 
which Clang versions support that option?

Also, since 'configure' is already testing for which warning options 
work by invoking the compiler directly, perhaps this part of 
gnulib-common.m4 could leverage off that work rather than trying to 
hardcode which compilers support which options.

I'll cc this to bug-gnulib for further comment there.