[gjs/mcatanzaro/#336] Fix build on 32-bit architectures

[Michael Catanzaro <mcatanzaro src gnome org>]

commit ec20c4783de93f9b31f37c787a35d5cd602c9fec
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Mon Aug 3 16:15:31 2020 -0500

    Fix build on 32-bit architectures
    
    Fixes #336

 gi/arg-inl.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)
---
diff --git a/gi/arg-inl.h b/gi/arg-inl.h
index 7a109ef4..7f313769 100644
--- a/gi/arg-inl.h
+++ b/gi/arg-inl.h
@@ -113,7 +113,17 @@ GJS_USE inline decltype(auto) gjs_arg_member<char32_t>(GIArgument* arg) {
 template <>
 GJS_USE inline decltype(auto) gjs_arg_member<GType, GI_TYPE_TAG_GTYPE>(
     GIArgument* arg) {
-    return gjs_arg_member(arg, &GIArgument::v_size);
+    // GType is defined differently on 32-bit vs. 64-bit architectures. From gtype.h:
+    //
+    // #if     GLIB_SIZEOF_SIZE_T != GLIB_SIZEOF_LONG || !defined __cplusplus
+    // typedef gsize                           GType;
+    // #else   /* for historic reasons, C++ links against gulong GTypes */
+    // typedef gulong                          GType;
+    // #endif
+    if constexpr (sizeof(gsize) != sizeof(gulong))
+        return gjs_arg_member(arg, &GIArgument::v_size);
+    else
+        return gjs_arg_member(arg, &GIArgument::v_ulong);
 }
 
 template <>