build: prevent dupes in autotools.mk

build: prevent dupes in autotools.mk

autotools.mk does not have any protection currently that would prevent
it from being sourced multiple times. Note that both package.mk and
host-build.mk source autotools.mk. So any package Makefile that includes
both will cause hooks to be added twice (at least twice).

This is fixed by declaring a new variable, __autotools_inc, and only
continuing if this variable doesn't equal 1. The same is done by
rules.mk already.

Also, this commit does away with an ifneq that checks PKG_FIXUP (instead
of HOST_FIXUP) for patch-libtool before adding to the host pre-configure
hook. This does not make sense.

The second ifneq is amended. The current one manually does what the
define patch_libtool_host is already doing. It can just use the define.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>