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/FindWAYLANDEGL.cmake

@@ -0,0 +1,18 @@
+find_package(PkgConfig)
+pkg_check_modules(WAYLANDEGL QUIET wayland-egl)
+
+set(VERSION_OK TRUE)
+if (WAYLANDEGL_VERSION)
+    if (WAYLANDEGL_FIND_VERSION_EXACT)
+        if (NOT("${WAYLANDEGL_FIND_VERSION}" VERSION_EQUAL "${WAYLANDEGL_VERSION}"))
+            set(VERSION_OK FALSE)
+        endif ()
+    else ()
+        if ("${WAYLANDEGL_VERSION}" VERSION_LESS "${WAYLANDEGL_FIND_VERSION}")
+            set(VERSION_OK FALSE)
+        endif ()
+    endif ()
+endif ()
+
+include(FindPackageHandleStandardArgs)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(WAYLANDEGL DEFAULT_MSG WAYLANDEGL_LIBRARIES VERSION_OK)