cmake: fix absolute path to GL libraries in pxrConfig.cmake

[pmolodo]

Description of Change(s)

The absolute path to GL makes pxrConfig.cmake non-portable.

Fixes Issue(s)

• can't use pxrConfig.cmake on computers where location of GL library differs.

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement

[jesschimein]

Filed as internal issue #USD-8982

[pmolodo]

I've confirmed that an absolute path sneaks in on Linux (tested on Ubuntu 22.04) - tried on Windows, and apparently it's not an issue there, for whatever reason.

Unsure of behavior on MacOS...

[tallytalwar]

On MacOS I think "OPENGL_gl_LIBRARY" gets substituted by "-framework OpenGL" explicitly... #2772, but I think like you said linking to "OpenGL::GL" target should also work on MacOS.

Also will go ahead and apply the same fix for pxr/imaging/glf/CMakeLists.txt

[pmolodo]

Actually - now that the Metal storm delegate exists, does MacOS even use / link to OpenGL anymore?

[tallytalwar]

Hi @pmolodo , I confirmed with David and our imaging tests still depend on GL (via garch and glf) even on macOS. So I believe we still need this. The change is checked in internally and will be available on next dev push.

[pmolodo]

Awesome, good to hear!