fixup: always use correct path for automake/aclocal wrappers

fixup: always use correct path for automake/aclocal wrappers

Otherwise, bootstrapping from a git checkout won't work anymore.
Issue introduced in today's commit 'v1.11-1974-ga98e58f'.

* doc/Makefile.am ($(srcdir)/amhello-1.0.tar.gz): Export PATH in a
way that makes the automake and aclocal wrappers truly accessible
(the location of this wrappers has been modified in a recent commit,
but this makefile hadn't been updated accordingly).
(update_mans): Likewise.