Compiling with CMake for Windows/MSVC only supports DLL runtime libraries

I'm attempting to consume opus as a git submodule from an official tag, and build a Godot extension cross-compiling for Windows, Linux, and Android.

The current CMakeLists.txt targets cmake 3.1 which hard-codes the C runtime library to either MultiThreadedDebugDLL or MultiThreadedDLL. The resulting code therefore has to ship the Microsoft runtime library DLL.

Upgrading CMakeLists.txt to cmake_minimum_required(VERSION 3.15) unlocks the options for specifying the runtime library options on the command-line such as cmake -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded .

It would be preferable to support an OPUS_STATIC_RUNTIME option, which for MSVC would use CMAKE_MSVC_RUNTIME_LIBRARY Generator Expressions to correctly specify the Debug or Release Static or DLL runtimes.

Possibly something like:

if(MSVC)
  if(OPUS_STATIC_RUNTIME)
    set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
  else()
    set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>DLL")
  endif()
endif()

Start in CMakeLists.txt and review the existing CMake 3.1 runtime configuration alongside CMAKE_MSVC_RUNTIME_LIBRARY and its generator-expression documentation. Configure Debug and Release MSVC builds with and without OPUS_STATIC_RUNTIME; done means both static and DLL runtime selections work without changing the non-MSVC build paths.