When building on a newer host, boostrap using lex from the stable-8 source
to match the yacc being used from that source. This avoids a build error
caused by the newer lex emitting a yylex() decl that's already in the source.

This is a direct commit to stable-8; there is no corresponding change in
later branches to MFC from. For the record, the new lex came in at version
1000032, but slipping it into the 1000013 block makes more sense than
creating a whole new .if block for it.