Port build procedure to AIX 7.1

Port build procedure to AIX 7.1

* lib/freeze.mk (MY_AUTOM4TE, build_libdir, m4f_dependencies):
Prefer ‘$(top_build_prefix)’ to ‘$(top_builddir)/’.
The difference matters on AIX 7.1, where ‘make’ doesn’t know that
bin/autom4te and ./bin/autom4te are the same file,
and gets confused about dependencies without this change.
‘$(top_build_prefix)bin/autom4te’ expands to ‘bin/automake’
whereas ‘$(top_builddir)/bin/autom4te’ expands to
‘./bin/automake’, and the former works where the latter doesn’t.