bootstrap/contrib: use our locally built libtool via ACLOCAL_PATH

This is the proper variable to use when setting the path to custom build autotools. It still lets any -I call to autoreconf to overwrite any folder.

Forcing an ACLOCAL_AMFLAGS environment value is wrong. It can be overriden in contribs. That's why the same values were also passed in the command line in contribs. But then it messes up the priority in which autotools macros are looked into.

Based on top of !3992 (closed).