glib/gmacros.h: Move <type_traits> include to consumers

When included inside an `extern "C"` block, this causes build failures
that look something like:

  /usr/include/c++/10/type_traits:2930:3: error: template with C linkage
   2930 |   template<typename _Fn, typename... _Args>
        |   ^~~~~~~~
  ../../disas/arm-a64.cc:20:1: note: ‘extern "C"’ linkage started here
     20 | extern "C" {
        | ^~~~~~~~~~

Commit 4273c43902 made this opt in for
projects which are defining `GLIB_VERSION_MIN_REQUIRED`, but the include
of `<type_traits>` via `gmacros.h` was not included in this. If we move
the include out to the places where `glib_typeof` is called, we can make
it covered by this macro too, and save a few consumers from FTBFSing.

That also means that, if you don't want to fix your use of the headers,
and as long as this version is sufficient for you, a quick workaround is
to define `GLIB_VERSION_MIN_REQUIRED` to `GLIB_VERSION_2_66` or lower.

Suggested by Simon McVittie.

Alternative to: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1935
Fixes: https://gitlab.gnome.org/GNOME/glib/-/issues/2331

glib/gatomic.h

@@ -26,6 +26,11 @@
 
 #include <glib/gtypes.h>
 
+#if defined(glib_typeof_2_68) && GLIB_VERSION_MIN_REQUIRED >= GLIB_VERSION_2_68
+/* for glib_typeof */
+#include <type_traits>
+#endif
+
 G_BEGIN_DECLS
 
 GLIB_AVAILABLE_IN_ALL

glib/gmacros.h

@@ -238,7 +238,12 @@
 #define glib_typeof(t) __typeof__ (t)
 #elif defined(__cplusplus) && __cplusplus >= 201103L
 /* C++11 decltype() is close enough for our usage */
-#include <type_traits>
+/* This needs `#include <type_traits>`, but we have guarded this feature with a
+ * `GLIB_VERSION_MIN_REQUIRED >= GLIB_VERSION_2_68` check, and such a check
+ * cannot be enforced in this header due to include ordering requirements.
+ * Within GLib itself, which use `glib_typeof` need to add the include
+ * themselves. See other examples in GLib for how to do this.
+ */
 #define glib_typeof(t) typename std::remove_reference<decltype (t)>::type
 #define glib_typeof_2_68
 #endif

glib/gmem.h

@@ -31,6 +31,11 @@
 
 #include <glib/gutils.h>
 
+#if defined(glib_typeof_2_68) && GLIB_VERSION_MIN_REQUIRED >= GLIB_VERSION_2_68
+/* for glib_typeof */
+#include <type_traits>
+#endif
+
 G_BEGIN_DECLS
 
 /**

glib/grcbox.h

@@ -24,6 +24,11 @@
 
 #include <glib/gmem.h>
 
+#if defined(glib_typeof_2_68) && GLIB_VERSION_MIN_REQUIRED >= GLIB_VERSION_2_68
+/* for glib_typeof */
+#include <type_traits>
+#endif
+
 G_BEGIN_DECLS
 
 GLIB_AVAILABLE_IN_2_58

gobject/gobject.h

@@ -28,6 +28,11 @@
 #include        <gobject/gsignal.h>
 #include        <gobject/gboxed.h>
 
+#if defined(glib_typeof_2_68) && GLIB_VERSION_MIN_REQUIRED >= GLIB_VERSION_2_68
+/* for glib_typeof */
+#include <type_traits>
+#endif
+
 G_BEGIN_DECLS
 
 /* --- type macros --- */