Fix linking issues with wxGLCanvasEGL for cmake

1) Link with EGL and wayland-egl libraries (with check for GDK Wayland).
2) Provide a way to force disable the EGL backend.

build/cmake/modules/FindGTK3.cmake

@@ -42,5 +42,9 @@ set(VERSION_OK FALSE)
 endif ()
 endif ()
 endif ()
+# Check for GDK Wayland support
+include(CheckSymbolExists)
+set(CMAKE_REQUIRED_INCLUDES ${GTK3_INCLUDE_DIRS})
+check_symbol_exists(GDK_WINDOWING_WAYLAND "gdk/gdk.h" wxHAVE_GDK_WAYLAND)
 include(FindPackageHandleStandardArgs)
 FIND_PACKAGE_HANDLE_STANDARD_ARGS(GTK3 DEFAULT_MSG GTK3_INCLUDE_DIRS GTK3_LIBRARIES VERSION_OK)