Remove possible warnings with redefinitions

[Uran198]

If the library is used as part of a bigger project some definitions can
be provided via external build files.

[lewissbaker]

So the issue you're trying to solve here is that you're already defining WIN32_LEAN_AND_MEAN on the command-line in your build system which is compiling cppcoro sources directly rather than using the output of the cake build system?

There are a couple of new files in the sockets2 branch that may require compilation without WIN32_LEAN_AND_MEAN defined. I'll try out this change with those branches but otherwise it seems like a reasonable change to me.

[Uran198]

So the issue you're trying to solve here is that you're already defining WIN32_LEAN_AND_MEAN on the command-line in your build system which is compiling cppcoro sources directly rather than using the output of the cake build system?

Yes, exactly.