This way we can enclose some defines in an ifdef block, in order not to
override some values which could be provided through the command line.
This aims at fixing UWP builds, which have a specific _WIN32_WINNT value
provided through the command line, while enforcing our default value
would cause many APIs to be unavailable

It's not tested anymore, and grows out of sync each time a file gets
added/removed
In addition, we'll soon need to provide a config.h.in to meson in order
to be able to define configuration macros in an ifdef block so we might
as well not let that file be overriden by autoconf