flex: needs M4 at runtime

For proper runtime execution, flex requires m4 to be
installed. Passing a M4 variable at configure time is needed,
otherwise flex on the target will try to use a 'm4' binary with a
build machine path.

Fixes bug #4988.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>