Ignore redundant-decls warning on MinGW

[AMS21]

Description

What

Added warning suppression for -Wredundant-decls on MinGW.

Why

When compiling with MinGW and in your test file you include windows.h first and then doctest.h, you'll get this -Wredundant-decls warning. Another solution would be to include windows.h to silence the warning. But I think that wouldn't be preferable. Link to CI build displaying the Problem

GitHub Issues

N/A

[onqtam]

Thanks for the fix! Strange that this doesn't happen in the doctest CI builds... Merging it.
Note that since this commit doesn't regenerate the final doctest.h this fix won't be immediately available in the dev branch. For regenerating the final header CMake must be invoked and a build should take place (there is a CMake custom command for that).