Use framework linkage for OpenGL on macOS

[dgovil]

This commit fixes the output of the MaterialXTargets.cmake to have proper system linkage for OpenGL in the cmake config.
Without this change, downstream projects on a Mac like USD cannot build against MaterialX properly.

There was an issue introduced in b50dea8 where the output started being like so

set_target_properties(MaterialXRenderGlsl PROPERTIES
  INTERFACE_COMPILE_DEFINITIONS "GLAD_GLAPI_EXPORT"
  INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
  INTERFACE_LINK_LIBRARIES "MaterialXRenderHw;MaterialXGenGlsl;OpenGL::GL;-framework Foundation;-framework Cocoa;-framework Metal"
)

Prior to that commit it would end up being a hardcoded path to the SDK which is also not great, because it means any SDK update would make the file unusable.

/Library/Developer/CommandLineTools/SDKs/MacOSX13.5.sdk/System/Library/Frameworks/OpenGL.framework;

Switching to -framework like the rest of the system links fixes that and makes it portable regardless of what SDK paths are on a system

[jstone-lucasfilm]

This looks like a good improvement, thanks @dgovil!

Since we're currently focusing on MaterialX 1.39 for release this summer, would you mind retargeting this pull request to the dev_1.39 branch, and we can review it there?

[dgovil]

Changed the target to dev_1.39, though I do think the bug its fixing should be considered a breaking change in terms of letting other projects compile against MaterialX 1.38.

I'm not sure if there's room for patch updates, like a 1.38.9.1 or just a bug fix 1.38.10 release to back port fixes?
If not for this, I think it would make sense in the future just to alleviate possible issues for people who aren't ready to make as big a jump up.

[jstone-lucasfilm]

@dgovil It should be fine to create a new 1.38.10 release as well, if there are important fixes that need to be included in the 1.38.x series, but so far we've been focused on the 1.39 release this summer. Let us know if you believe this change is important enough to warrant an additional 1.38.10 release, and we can bring that up in the next MaterialX TSC.

[dgovil]

I think it might be worth having a 1.38.10 to back port some fixes like this. My worry is this one makes it harder for macOS devs to use 1.38.9, and I'm not sure they'd be ready to move to 1.39

I can bring it up in the next TSC meeting.

[jstone-lucasfilm]

This looks good to me, thanks @dgovil!