Re: pg_attribute_noreturn(), MSVC, C11

I committed the first two patches (squashed together) (about 
pg_noreturn).  I had to make one change: I put back the GCC fallback 
that I had removed between v1 and v2.  This is needed for GCC versions 
before C11 became the default (gcc 5) and also for situations like 
buildfarm member mylodon that builds with -std=c99 explicitly. 
(Otherwise, that configuration would get a bunch of compiler warnings 
about uninitialized variables etc.)  I also added the additional comment 
about placement that you had requested.

I'm going to postpone the remaining two patches (about pg_nodiscard). 
After experimenting a bit more, I'm less sure about what the correct 
placement of C23 attributes is meant to be, and without understanding 
that, I fear this would make the earlier question about the correct 
placement of pg_noreturn unnecessarily more complicated.  This can be a 
future project.