changes flex command from lex to flex by hard code:
# lib/libc/net Yacc and lex files
lib/libc/net/nslexer.c: lib/libc/net/nslexer.l
	cd lib/libc/net && flex nslexer.l

lib/libc/net/nsparser.c: lib/libc/net/nsparser.y
	make -C lib/libc/net nsparser.c

# libipsec Yacc and lex files
lib/libipsec/policy_token.c: lib/libipsec/policy_token.l
	cd lib/libipsec && flex policy_token.l

lib/libipsec/policy_parse.c: lib/libipsec/policy_parse.y
	make -C lib/libipsec policy_parse.c