Re: change "microcode: Use fallthrough attribute on clang 10 and 11 too." broke the build for me

[Taylor R Campbell]

> Date: Sun, 14 Feb 2021 16:10:35 -0800
> From: Chris Hanson <cph@chris-hanson.org>
> References: <20210214221024.50A4F60858@jupiter.mumble.net>
> 
> OK, we have to treat the Apple clang differently.

That seems reasonable -- I suggested

#define FALLTHROUGH() [[fallthrough]]

under #ifdef __APPLE__ but I don't have a way to test and I forget
whether that patch worked for Arthur.

> Prior to v12 no attribute is necessary as there are no warnings for 
> falling through. Starting with v12 the usual attribute is required and 
> works.

That may be accurate on macOS, but it does not seem to be accurate in
general -- this is the report with clang 10 that motivated me to the
reduce __clang_major__ >= 12 test to __clang_major__ >= 10:

https://lists.gnu.org/archive/html/mit-scheme-devel/2021-02/msg00004.html

> I'll make the change and cherry-pick to release-11, unless you want to 
> do it.

Go for it!