cmake: older emscripten releases have a broken FindOpenGL.cmake

This is fixed since 3.1.10: https://github.com/emscripten-core/emscripten/commit/485a7b4d6f84d4bc3b594ff81d39b6b143c8d598#diff-034f4d123f23ec5493d0fbf28cba1c36e404a991f286c8d031a22799e4e8b0e5

test/CMakeLists.txt

@@ -230,6 +230,9 @@ if(OPENGL_FOUND)
         target_link_libraries(testshader OpenGL::GL)
         target_link_libraries(testgl2 OpenGL::GL)
     else()
+        if(EMSCRIPTEN AND OPENGL_gl_LIBRARY STREQUAL "nul")
+            set(OPENGL_gl_LIBRARY GL)
+        endif()
         # emscripten's FindOpenGL.cmake does not create OpenGL::GL
         target_link_libraries(testshader ${OPENGL_gl_LIBRARY})
         target_link_libraries(testgl2 ${OPENGL_gl_LIBRARY})