windows: Define WIN32_LEAN_AND_MEAN to make compilation faster.
windows.h includes a lot of other headers, slowing down compilation
significantly. WIN32_LEAN_AND_MEAN reduces that a bit. It'd be better to
remove the include of windows.h (as well as indirect inclusions of it) from such
a central place, but until then...
Discussion: https://postgr.es/m/
20210921193035.pqzay43vpyv7in43@alap3.anarazel.de