I noticed recently that Emacs didn't have any build instructions for 64-bit
Windows - I tried the instructions for the 32-bit build in nt/INSTALL last
year  but something went wrong at some point (I don't remember what,
exactly). I tried again recently using the instructions from the Emacs-w64
project  which uses MSYS2 and MinGW-w64 - it worked well and it was also
easier to get the necessary libraries, as it has a package manager, pacman.
So I thought I would try adapting those instructions to include with Emacs, if
it would be helpful - the author Chris Zheng said he wouldn't mind, and I could
post it here or the devlist for feedback - it could go in a separate INSTALL64
document, if the existing one is too long.
Are there any issues with using MSYS2 over MSYS? I think you can build a 32-bit
Emacs with MSYS2 also, but I'm not sure - if so it would be possible to cover
both with one document, which might be simpler, but I could start with the
64-bit instructions anyway.