Replace nslexer.l->nslexer.c custom rule with a -D CFLAG.

This avoids reproducing the lex logic which had dependencies set wrong
and used an intermediate file for modifying the YY_BUF_SIZE.

This has only been possible since flex 2.5.37 was imported in r250873,
which uses #ifndef YY_BUF_SIZE.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division