Fix CMake warning on macOS since CMake 3.9

[Ceylo]

This fixes this warning since CMake 3.9 where CMP0068 was introduced:

CMake Warning (dev):
  Policy CMP0068 is not set: RPATH settings on macOS do not affect
  install_name.  Run "cmake --help-policy CMP0068" for policy details.  Use
  the cmake_policy command to set the policy and suppress this warning.

  For compatibility with older versions of CMake, the install_name fields for
  the following targets are still affected by RPATH settings:

   SFML
   sfml-audio
   sfml-graphics
   sfml-network
   sfml-system
   sfml-window

This warning is for project developers.  Use -Wno-dev to suppress it.

Happens at CMake generation time.
Originally reported by @mantognini in #1344

I checked the output of the installed dylibs and frameworks with "otool -L", after having generated both with CMake 3.10 and 3.0, and they looked exactly the same as what's shipped for current latest SFML release.

[Foaly]

I gave this a quick test and it fixes the warning as expected (cmake 3.10.2). I couldn't test the actual install part as I am not quiet sure how.

[Ceylo]

To install you just need to build the “install” target. Just make sure to select a writable install prefix in CMake config.

About testing, I don’t think installing will help much. You can but you mainly need to check that the frameworks/dylibs can be embedded in bundle apps exactly the same way as previously, or check the paths shown by “otool -L” on all generated binaries. Probably the easiest and most reliable.

[Ceylo]

@mantognini Are you able to check the PR so that it can be merged? Dunno if you're still away from home though.

[JonnyPtn]

I've tested this and it does indeed make the warning go away. Doesn't seem to have any adverse affect on my game bundling either

[mantognini]

@Ceylo, do you prefer having this one merged first, or rather after the big one #1335? I'll try this one as soon as possible.

[Ceylo]

@mantognini I’d really prefer the big one to be merged first : it’ll fix CI, and some of my changes for iOS depend on it, and it’s much easier to merge and fix conflict in a small PR :)

[eXpl0it3r]

Merged in 8b7a50a

[ChrisThrasher]

Did we mean to use SFML here instead of ${target}? I can't find a variable defined with the name target in CMakeLists.txt. I just found this out because I started enforcing uninitialized variable warnings.

https://github.com/SFML/SFML/actions/runs/6859641809/job/18652242185#step:14:43