Update SDL2, zlib, freetype2 dependencies

[juliusikkala]

Recently, CMake broke compatibility with older versions, breaking compatibility with a bunch of projects. This also affected dependencies in PDCursesMod. This PR resolves the problem by simply updating those dependencies to their latest releases, which have fixed this issue on their end already.

The errors look like this:

CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

[GitMensch]

Not sure, but wouldn't it be reasonable to specify the minimal version that works instead of the latest one? After all it is a minimal dependency.

[juliusikkala]

The way these dependencies are currently set up is that they get downloaded and built during the cmake build (it's not even trying to find those libraries on the host), so using the latest version shouldn't be an issue IMO. Although, that being said, the updated freetype2 apparently no longer honors our bundled zlib and does try to find it from the host system, causing the CI fail. I'll take a look if I can do something about that. nevermind, upstream zlib just has changed the name of the library from libz.lib to z.lib on windows 😄