>> FYI, the claim "Sun C 5.8, a C99 compiler that does presently produce 
>> working Emacs builds" is pure fantasy, aka fabricated evidence.  I had 
>> some free time today, I digged in my archives, and found a copy of 
>> Solaris 10 and Sun Studio 11, which I installed in a VM.  The last 
>> Emacs version for which Sun C 5.8 produces a working Emacs build is... 
>> Emacs 24.
>
> Really?
>

Really.

>
> What's the error
>

It's not "the error", it's "the errors".  Building Emacs 25 to 29 with the 
simplest possible configuration (--without-all --without-x) fails, and 
fails early on (while compiling dispnew.c), but the error messages are 
different in each case.  I attach those of Emacs 25.

>
> and what CPU did you install it on?
>

That would have mattered only if the compiler had produced a non-working 
build.  It cannot even create the first object file.