Fix upgrading from OSVERSION 1000002-1000032 after r288829.

r288829 states that lex requires the latest m4, but was not always building it.
Move lex to the same logic as m4 since they are closely tied now.

MFC after: 3 days
Sponsored by: EMC / Isilon Storage Division
Reported by: Slawa Olhovchenkov <slw@zxy.spb.ru>