Add option to disable spawnv define

[JorisGoosen]

Helps UCRT mingw users workaround the problem mentioned in #300

[JorisGoosen]

@evanmiller ?

[evanmiller]

I would prefer to have the script detect the platform and do the right behavior, rather than add an option.

[JorisGoosen]

I would prefer to have the script detect the platform and do the right behavior, rather than add an option.

Fair enough, Ill have a look

[JorisGoosen]

The new configure doesnt require a manual option anymore, I tested it on Mingw x64 and Mingw UCRT x64 and it now configures and compiles on both without a hitch

[evanmiller]

There's probably a cleaner way to do this with autconf (see https://stackoverflow.com/questions/46704731/check-value-of-c-define-with-autoconf) but this seems good enough for now, thanks

[JorisGoosen]

Ah yeah that SO answer looks cleaner for sure, but thanks for merging it anyway. It will make our build process much smoother.