automake: use whenever possible the ACLOCAL_HOST_DIR variable

Use the ACLOCAL_HOST_DIR in the GTK_DOC_M4_INSTALL hook.

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

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>