Fix build with CMake 3.16

Use FindOpenGL from CMake upstream which finds both OpenGL and EGL.

Fix linking by using OpenGL::GLX (instead of ${OPENGL_gl_LIBRARY} which is
set to an empty string when GLVND is used). Thanks to Ömer Fadıl Usta for
mentioning that in https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/264

NO_CHANGELOG

CMakeLists.txt

@@ -102,12 +102,10 @@ set_package_properties(OpenGL PROPERTIES DESCRIPTION "The OpenGL libraries"
                        TYPE OPTIONAL
                       )
 
-find_package(EGL)
-set_package_properties(EGL PROPERTIES
-                       PURPOSE "Support for Window Thumbnail on EGL platform"
-                       TYPE OPTIONAL
-                      )
-set(HAVE_EGL ${EGL_FOUND})
+add_feature_info(EGL ${OpenGL_EGL_FOUND}
+                 "A platform-agnostic mechanism for creating rendering surfaces for use with other graphics libraries, such as OpenGL|ES and OpenVG.")
+# OpenGL_EGL_FOUND is defined by FindOpenGL
+set(HAVE_EGL ${OpenGL_EGL_FOUND})
 
 if(OPENGL_FOUND AND (${Qt5Gui_OPENGL_IMPLEMENTATION} STREQUAL "GL"))
     set(HAVE_GLX ${HAVE_X11})

src/declarativeimports/core/CMakeLists.txt

@@ -53,12 +53,12 @@ if(HAVE_X11)
   endif()
 
   if(HAVE_GLX)
-    target_link_libraries(corebindingsplugin ${OPENGL_gl_LIBRARY})
+    target_link_libraries(corebindingsplugin OpenGL::GLX)
   endif()
 endif()
 
 if(HAVE_EGL)
-    target_link_libraries(corebindingsplugin EGL::EGL)
+    target_link_libraries(corebindingsplugin OpenGL::EGL)
 endif()
 
 if (${Qt5Gui_OPENGL_IMPLEMENTATION} STREQUAL "GLESv2")