automake: use whenever possible the ACLOCAL_DIR variable

Use the ACLOCAL_DIR in the GTK_DOC_M4_INSTALL hook.

Move the ACLOCAL_DIR definition at the beginning so the definition is
ahead the utilization.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>