Support Sf2 Player, Mallets, PortAudio, and Lame with MSVC

[DomClark]

As in #6132, this is done by modifying the CMake code to consume these as imported targets, which can come either from vcpkg or from our own find modules. The FluidSynth port in vcpkg doesn't come with CMake support, but I have written a find module that produces an imported target anyway, as this is the preferred modern approach.

Changes:

• Added a find module for FluidSynth.
• Modified the find modules for STK, PortAudio, and LAME to create imported targets, first checking whether config mode produces such a target already.
• Moved the config mode preference logic for SDL2 from the root CMakeLists.txt to the find module, for consistency with the others. I feel this is better than cluttering the list file with logic related to finding packages, which is the whole point of find modules.
• Cleaned up the SDL2 find module since I was working on it anyway. I've removed all the unused bits, and modernised the code style.
• Set the sndio status for Windows builds to our standard unsupported platform message; previously it was blank, unlike all of the other optional feature statuses.
• Introduced a helper function for finding the appropriate static (import) libraries in vcpkg for different build types. This is useful when CMake integration is not provided. I used it for FluidSynth, but it could be useful for FFmpeg, for example.
• Added specific search hints for MinGW to SDL2. The config mode script for SDL2 on MinGW doesn't provide a target, but does offer directories to look in. We might as well take advantage of those.
• Set the host triplet for vcpkg to match the target triplet. Building FluidSynth requires building a code generation tool to generate some headers, and that tool shares a number of dependencies with FluidSynth. Matching the host and target triplets means only one version of these dependencies needs to be built, which cuts down the build time significantly.
• Set CMAKE_NO_SYSTEM_FROM_IMPORTED for MinGW builds. Some headers of libstdc++ are sensitive to the include search order, which can be changed by specifying system headers. Treating imported target headers as standard includes avoids modifying the order inadvertently.

I think this only leaves the LADSPA plugins, ZynAddSubFx, and Gig Player missing from MSVC builds now.