/O2 in CMakeLists.txt is incompatible with vcpkg using Ninja

[davidmatson]

I'm updating the blingfire port in vcpkg so that it can work with uwp projects, and the /O2 switch caused a problem I had to patch:
https://github.com/microsoft/vcpkg/pull/31331/files#diff-c5ec2f3e81e4876b6c3da4509ba47620f5b1f2bca2cd50ce1a228ced2f91fa96

Can we make a fix here so that this patch isn't needed long-term? I'm not sure how optimization switches are normally done in CMake, especially when using Ninja.