Revert "meson: simplify builtin_immodules build option"

This reverts commit 749a58ab26.

This maintains feature parity with the autotools build. To achieve the
equivalent of --with-included-immodules=wayland,
-Dbuiltin_immodules=wayland,waylandgtk can be used instead.

gtk/meson.build

@@ -959,7 +959,7 @@ foreach l: immodules
   cond = l.get(2, true)
   cflags = l.get(3, [])
 
-  if cond and builtin_immodules
+  if cond and (builtin_immodules.contains(name) or builtin_all_immodules)
     gtk_cargs += ['-DINCLUDE_IM_@0@'.format(name.underscorify())]
     mod = static_library('staticimmodule-@0@'.format(name),
                          sources + gtk_dep_sources,