gutils: Add internal API to override XDG directories

Add a new internal function, g_set_user_dirs(), which will safely
override the values returned by g_get_user_data_dir() and friends, and
the value returned by g_get_home_dir().

This is intended to be used by unit tests, and will be hooked up to them
in a following commit.

This can be called as many times as needed by the current process. It’s
thread-safe. It does not modify the environment, so none of the changes
are propagated to any subsequently spawned subprocesses.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/538

docs/reference/glib/Makefile.am

@@ -64,6 +64,7 @@ IGNORE_HFILES = \
 	glib-init.h		\
 	gconstructor.h		\
 	valgrind.h		\
+	gutilsprivate.h		\
 	gvalgrind.h		\
 	$(NULL)
 

docs/reference/glib/meson.build

@@ -36,6 +36,7 @@ if get_option('gtk_doc')
     'glib-init.h',
     'gconstructor.h',
     'valgrind.h',
+    'gutilsprivate.h',
     'gvalgrind.h',
   ]