[Not exactly the same as building with the MinGW compiler, but some
input on how it might be possible to build on Windows using autotools,
but with a Microsoft compiler (cl.exe) instead of the MinGW compiler.]
It's been a very long time since I looked at this problem space, so the
things discussed in the below thread might have bit-rotted in all sorts
of spectacular manners. But, for what it's worth:
https://lists.gnu.org/archive/html/automake/2011-09/msg00003.html
The compile and ar-lib scripts mentioned are in upstream automake since
many years, so the variant with just AR=lib should be OK since I see
that AM_PROG_AR has already been added to the NTP configure.ac.
Who knows, this might be a way forward? I'm of course not holding my
breath, and sadly don't really have any time to spend...