lex.prf was trying to be halfway between the standard POSIX lex
requirements and those of GNU flex. So fix it to work with both, more or
less, by noticing when lex is actually flex and using the extended GNU
options. Note that POSIX lex is untested and may still not work.

Change-Id: Ib306f8f647014b399b87ffff13f1e8e43fb68b3c
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>