build: Use build-aux for auxiliary build scripts

This is the name most projects have settled on, so let's use that
as well.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/406>

data/meson.build

@@ -23,7 +23,7 @@ foreach name : session_desktops
     session_instdir = wlsessiondir
   else
 # FIXME: The same target can not be copied into two directories.
-    #        There is a workaround in meson/session-post-install.py until proper
+    #        There is a workaround in build-aux/session-post-install.py until proper
     #        solution arises:
     #        https://github.com/mesonbuild/meson/issues/2416
     session_instdir = wlsessiondir