Fix the fix for G*_TO_POINTER casts on 32 bits

The 'fix' from commit 16292dd753 broke the
build on 32 bit because it was missing the parentheses around "gint" / "guint"
in glib_gp[u]i_cast.

https://bugzilla.gnome.org/show_bug.cgi?id=661546

configure.ac

@@ -3360,8 +3360,8 @@ $ac_cv_sizeof_int)
   gintptr_modifier='""'
   gintptr_format='"i"'
   guintptr_format='"u"'
-  glib_gpi_cast='gint'
-  glib_gpui_cast='guint'
+  glib_gpi_cast='(gint)'
+  glib_gpui_cast='(guint)'
   ;;
 $ac_cv_sizeof_long)
   glib_intptr_type_define=long