Support LV2 with MSVC

[DomClark]

Allows building LV2 support for Windows using MSVC. Adds the required vcpkg ports to .appveyor.yml, changes CMakeLists.txt to be able to use these ports, and makes a few small changes required to successfully compile under MSVC.

[LmmsBot]

🤖 Hey, I'm @LmmsBot from github.com/lmms/bot and I made downloads for this pull request, click me to make them magically appear! 🎩

Linux

• Linux (AppImage): lmms-1.2.3-743%2Bg4a1263d-linux-x86_64.AppImage (build link)

Windows

• Windows 32-bit: lmms-1.2.3-743%2Bg4a1263d79-mingw-win32.exe (build link)
• Windows 64-bit: lmms-1.2.3-743%2Bg4a1263d79-mingw-win64.exe (build link)
• Windows 32-bit: lmms-1.2.2-msvc2017-win32.exe (build link)
• Windows 64-bit: lmms-1.2.2-msvc2017-win64.exe (build link)

macOS

• macOS : lmms-1.2.3-743%2Bg4a1263d79-mac10.13.dmg (build link)

🤖

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[superpaik]

I don't know if that is correct for testing, but I installed the Windows 64-bit made by the boot and lv2 pluggins are not available. On the LMMS starting page you can read "Lv2 plugin SUMMARY: 0 of 0 loaded in 0 msecs"

[DomClark]

LMMS doesn't ship with any LV2 plugins by default. You can install some free ones for Windows from https://x42-plugins.com/x42/, for example.

[superpaik]

Sorry, my fault.
I installed a couple of effects (using "Windows 64-bit: lmms-1.2.2-msvc2017-win64.exe" for testing) and it worked ok.

[JohannesLorenz]

I left only one minor comment about the STATUS_LV2 message in CMake. LGTM.

[DomClark]

Will merge in a day or two if there are no objections.

[DomClark]

Merge?

[JohannesLorenz]

@DomClark for your info (it's not a comment stopping this PR): We have a new Lv2 wiki page. If compiling LMMS with Lv2 (or installing Lv2) is nontrivial, you can add the steps there. Also, you can add instructions how to install the plugins on Windows, if it's not obvious.

[tresf]

Merge?

Yes. I pointed out some wording that seems to be geared towards systems without pkg-config, which I believe @JohannesLorenz originally authored when suil was added in 2a66e83, but shouldn't hold this up. I'm just a bit confused who this wording was intended to target since pkg-config has been a requirement of LMMS as long as I can remember. :)

[DomClark]

@JohannesLorenz Thanks for the wiki link - I'll add the relevant information once this is merged. Compiling with LV2 should be easy (just vcpkg install lilv lv2), but the ports are quite new so anyone who's compiled LMMS on Windows before may need to update vcpkg first. Installing LV2 plugins is easy enough - the hard part is finding them, since it's not a common format on Windows and there's no package manager to search for them. I'll add a link to x42's plugins to get people started.