Simplify CMake Policies

[tresf]

Per #6758 (comment)

Notes:

• Checking for COMMAND CMAKE_POLICY seems redundant.
• Checking for CMAKE >= 3 is also redundant.
• Add comments per request in Enable LADSPA plugins on MSVC #6758 (comment)

[DomClark]

We require a minimum of CMake 3.9 now, which means all of these policies will use the new behaviour by default (since CMake 3.9 includes the policies up to and including CMP0069). Thus the only line that still has an effect is the one that sets CMP0050 to OLD. Furthermore, I don't think we require that old behaviour any more: the policy value was introduced in #3583 because the old behaviour was used in the RemoteVstPlugin build scripts, but that use was since removed in ea15469. I can't find any other uses of the old behaviour, and LMMS configures and builds correctly without it (at least on my machine, using MSVC).

There are a couple of other cmake_policy commands elsewhere in the project. There are two in cmake/modules/InstallDependencies.cmake, which are also obsolete. Another is in plugins/CarlaBase/CMakeLists.txt, which already has a decent comment above it. Finally, there's one in cmake/modules/DefineInstallVar.cmake, which is still required, and lacks a comment.

[tresf]

Thanks @DomClark. For the sake of consistency and setting the standard, I'm including the vertabitim descriptions next to the policy, even for those that are seemingly obvious.

All other requests should be addressed.

I've added a placeholder in the main CMakeLists.txt file, since it'll help the rebase for when #6758 is merged, and will make it more obvious where this policy goes per #6758 (comment).

[PhysSong]

We now require CMake 3.9, so isn't it possible to either remove the if or switch to the NEW behavior?

[tresf]

This wasn't merged, I mucked something up. 🤦 fixing now.

[tresf]

@PhysSong said:

👀

Yeah, pushed to the wrong branch 🤦. I disabled branch protection, reverted, just gotta use a new PR now.

New PR here: #6780