CMake: suppress OpenGL deprecation warnings on macOS Mojave

A single warning at configuration time is enough.

src/CMakeLists.txt

@@ -73,6 +73,10 @@ if(${PLATFORM} MATCHES "Desktop")
     find_library(OPENGL_LIBRARY OpenGL)
     set(LIBS_PRIVATE ${OPENGL_LIBRARY})
     link_libraries("${LIBS_PRIVATE}")
+    if (NOT CMAKE_SYSTEM STRLESS "Darwin-18.0.0")
+      add_definitions(-DGL_SILENCE_DEPRECATION)
+      MESSAGE(AUTHOR_WARNING "OpenGL is deprecated starting with macOS 10.14 (Mojave)!")
+    endif()
   elseif(WIN32)
     add_definitions(-D_CRT_SECURE_NO_WARNINGS)
   else()