At the same time, allow to enable jemalloc 4 with --enable-jemalloc=4.
MOZ_JEMALLOC4 will be deprecated later.
This also changes the semantics for freebsd, where the system jemalloc
is used, relying on MOZ_MEMORY being unset (default on freebsd) and
MOZ_JEMALLOC4 to be set. In this new setup, MOZ_JEMALLOC4 implies
--enable-jemalloc=4, which still works because of the corresponding
changes to old-configure.