> The Autotools docs say the triplet is
> cpu-vendor-os. They don't say anything about a version number.

Nevertheless Andreas is right, as the 'os' string is 'solaris2.11' in Autoconf 
triplets. I disagreed with this kind of 'os' string when it was introduced in 
the 1990s and I still think it's confusing, but it's been that way for decades 
and it's not likely to be worth changing now.

Also, Andreas is right that the best way to build 64-bit Emacs executables on 
Solaris is to configure with CC='gcc -m64' or CC='cc -m64' or whatever. This has 
long been documented in Emacs's etc/MACHINES file, and this method works for 
many other GNU packages as well. (If method doesn't work on some other packages, 
I suggest the problem lies with those other packages. :-)

That being said, it would be helpful for Emacs to cater to nonstandard 'os' 
strings like plain 'solaris', so I installed the attached patch into Emacs 
master to do that. This should appear in Emacs 28 whenever that comes out, but 
it's too risky to slide into Emacs 27 so when building that I suggest using 
'solaris2.11'.