Update CMake to 3.16 and add OPUS_STATIC_RUNTIME option

[Malcolmnixon]

This resolves issue #333 by:

• Updating CMake to 3.16 as recommended in the issue discussions
• Adding the OPUS_STATIC_RUNTIME option when using MSVC
• Updating the CMake CI action to use 3.16

[xnorpx]

@jmvalin can you hit approve and run se we can see the CI run.

[xnorpx]

@Malcolmnixon I assume you tested to build the static version in your project and verified it worked on a machine without the runtime dlls installed?

[Malcolmnixon]

@Malcolmnixon I assume you tested to build the static version in your project and verified it worked on a machine without the runtime dlls installed?

Yes - I confirmed cmake -DOPUS_STATIC_RUNTIME=ON produces a vcxproj with the runtime library configured for static (non-DLL) runtime, and the subsequent build then links correctly with my project.

[xnorpx]

@jmvalin this change LGTM but should just run it through CI once before merging.

[jmvalin]

Merged