Disable OpenGL functions deprecation warnings in wxOSX

Don't generate dozens of deprecation warnings (one for each OpenGL
function used in the code) for any program using wxGLCanvas being built
using macOS 10.14 or later SDK.

Closes https://github.com/wxWidgets/wxWidgets/pull/1875

include/wx/osx/glcanvas.h

@@ -16,6 +16,9 @@
 #import <OpenGLES/ES1/glext.h>
 #define wxUSE_OPENGL_EMULATION 1
 #else
+#ifndef GL_SILENCE_DEPRECATION
+    #define GL_SILENCE_DEPRECATION
+#endif
 #include <OpenGL/gl.h>
 #endif