In modern Windows code WIN32_LEAN_AND_MEAN should be defined.
Without this define windows.h pulls in some legacy headers, notably
winsocks.h. Modern code that cares about winsocks includes winsocks2.h
which conflicts with winsocks.h. Other code that does not care about
winsocks (OpenGL, fontconfig, ...) includes windows.h pulling in legacy
winsocks.h, causing definition conflicts and build errors.
At worst defining WIN32_LEAN_AND_MEAN requires explicitly including some
additional headers on Windows.
Michal Suchanek