build: Set a default aux dir

build: Set a default aux dir

This moves all the auxiliary build scripts into a build-aux directory,
and fixes an issue with configure being unable to find scripts because
it tries to change to an empty directory to get the absolute path,
which results in getting the path to the user's home directory instead.

,--
checking whether build environment is sane... yes
/bin/bash: /home/user/missing: No such file or directory
configure: WARNING: 'missing' script is too old or missing
`---

Signed-off-by: Guillem Jover <guillem@hadrons.org>