Link to the OpenGL::GL target, instead of ${OPENGL_LIBRARIES}

This is better because:

- targets are the modern way, and generally preferred
- provides an opportunity to inherit other properties of the target
- prevents absolute paths from creeping into the output
  MaterialXConfig.cmake, which makes it less portable
- ${OPENGL_LIBRARIES} includes libGLU (at least on linux), which is not
  actually a required dependency

source/MaterialXRenderGlsl/CMakeLists.txt

@@ -70,14 +70,14 @@ if(WIN32)
 elseif(APPLE)
     target_link_libraries(
         ${MATERIALX_MODULE_NAME}
-        ${OPENGL_LIBRARIES}
+        OpenGL::GL
         "-framework Foundation"
         "-framework Cocoa"
         "-framework Metal")
 elseif(UNIX)
     target_link_libraries(
         ${MATERIALX_MODULE_NAME}
-        ${OPENGL_LIBRARIES}
+        OpenGL::GL
         ${X11_LIBRARIES}
         ${X11_Xt_LIB})
 endif()

source/MaterialXRenderMsl/CMakeLists.txt

@@ -57,7 +57,7 @@ elseif(APPLE)
         target_link_libraries(
             ${MATERIALX_MODULE_NAME}
             "-framework Cocoa"
-            ${OPENGL_LIBRARIES}
+            OpenGL::GL
             )
     endif()
     target_link_libraries(
@@ -67,7 +67,7 @@ elseif(APPLE)
 elseif(UNIX)
     target_link_libraries(
         ${MATERIALX_MODULE_NAME}
-        ${OPENGL_LIBRARIES}
+        OpenGL::GL
         ${X11_LIBRARIES}
         ${X11_Xt_LIB})
 endif()