Add information about command path and version of flex in meson output

Hi all,

While doing a few things on Windows with meson, I have noticed that,
while we output some information related to bison after a setup step,
there is nothing about flex.

I think that adding something about flex in the "Programs" section
would be pretty useful, particularly for Windows as the command used
could be "flex" as much as "win_flex.exe".

Attached is a patch to show the path to the flex command used, as well
as its version.

Opinions or thoughts?
--
Michael