Allow checking for GDK backends

Allow checking for GDK backends

Now that a single shared object can contain multiple backends we also
need to provide a simple way for third party code to verify that the
copy of GDK they are linking to supports their backend.

The simplest way to verify is an m4 macro, GTK_CHECK_BACKEND(), shipped
with the gtk+ m4 macros.

The usage is pretty basic:

  GTK_CHECK_BACKEND([x11], [gtk_has_x11=yes], [gtk_has_x11=no])
  AM_CONDITIONAL(BUILD_X11_CODE, test "x$gtk_has_x11" = "xno")

https://bugzilla.gnome.org/show_bug.cgi?id=642479