Cleanup: bison and flex detection

The previous detection code was quite convoluted and required to have
two variables set to the same value to work. For example, the lexer in
autotools is configured through the LEX variable, even if we require a
specific implementation we have to use it and base our detection code on
it.

The situation is now :

  - the FLEX and BISON variables are ignored.
  - the LEX and YACC variables need to be set if required like before.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>