Do not hardcode USE_EXTERNAL_TINYXML2 to OFF on Windows

[traversaro]

Fortunately now in 2020 we have package managers on Windows, so at least giving an option to use an external TinyXML2 also on Windows make sense. : )

[traversaro]

The ignition_common-ci-pr_any-homebrew-amd64 failure seems to be unrelated.

[mjcarroll]

Do you think it makes sense to move this logic into ign-cmake so that it can be used in other places as well? I believe TinyXML2 is at least used here, in sdformat and a few more places up the stack.

[traversaro]

Do you think it makes sense to move this logic into ign-cmake so that it can be used in other places as well? I believe TinyXML2 is at least used here, in sdformat and a few more places up the stack.

I don't think so. USE_EXTERNAL_TINYXML2 is an option that only make sense for projects that vendor tinyxml2, and ignition-cmake does not provide any function to vendor tinyxml2 itself. Furthermore, given that package managers such as conan, vcpkg and conda are becoming to be more used on Windows, I hope that for future projects the need to vendor dependencies such as tinyxml2 will disappear.