meson: simplify builtin_immodules build option

Make it a yes/no/auto combo. "yes" means all modules are built into libgtk,
"no" that none are and "auto" uses the platform defaults, yes on win32,
no otherwise.

If we need more we can always extend it later.

gtk/meson.build

@@ -951,7 +951,7 @@ foreach l: immodules
   cond = l.get(2, true)
   cflags = l.get(3, [])
 
-  if cond and (builtin_immodules.contains(name) or builtin_all_immodules)
+  if cond and builtin_immodules
     mod = static_library('staticimmodule-@0@'.format(name),
                          sources + gtk_dep_sources,
                          dependencies: gtk_deps + [libgdk_dep] ,