windows build help

[phillip.lord@russet.org.uk (Phillip Lord)]

I was just reading 28601@debbugs.gnu.org. I have a slightly related
problem and wonder if anyone could help.

Currently, I am building the windows binaries for Emacs using an
out-of-source build with a directory structure like this:

git/emacs-26.0.50

build/emacs-26.0.50/x86_64
build/emacs-26.0.50/i686

install/emacs-26.0.50/x86_64
install/emacs-26.0.50/i686

This works nicely, but some of the process is manual. I wanted to script
everything up to and including the creation of the final zip files --
currently, the entire build takes about an hour or two.

But I do the x86_64 building using the MINGW64 shell
(i.e. C:\msys64\mingw64.exe) and the i686 using MINGW32
(i.e. C:\msys64\mingw64.exe). You might have thought I could just used
one -- the build for the i686 would just be cross compiling. But, this
doesn't work because I get this from ./configure:

checking for struct ifreq.ifr_addr.sa_len... no
checking whether gcc understands -MMD -MF... no
checking for long file names... yes
checking for X... no
checking whether Windows API headers are recent enough... no
configure: error: the Windows API headers are too old to support this build.

So, I cannot do both builds with a single command -- I have to come back
in the middle. I don't really understand the difference between the two
shells nor why I get this error.

Another solution would be to run both builds in parallel; this fails,
unfortunately, because the "out-of-source" build is not entirely out of
source -- the .elc files are created in-source. If I launch both a 64
and 32 bit build at the same time, they fight for whole compiles what,
until one or both crash.

It's not a major issue, but I'd quite like to start providing emacs-27
snapshots every couple of weeks. It's not going to happen, if it takes
too much effort.

Any thoughts welcome.

Phil