tests defs: allow requirements for compilers (mostly dummy)

tests defs: allow requirements for compilers (mostly dummy)

Most of the new requirements that are now accepted in `$required'
as consequence of this patch are still dummy.  They are planned
to be implemented only in master (or in some derived branch), but
having them here (even just as no-op) will allow for an easier
integration/backporting of potential new testcases.

* tests/defs.in (cc, c++, fortran, fortran77): New requirements,
still dummy.
(flex): New requirement, picking LEX for configure.
(lex): New requirement, alias for `flex'.  A more appropriate
implementation, looking for a generic `lex' program, will follow
in the future.
(yacc): New requirement, alias for `bison'.  A more appropriate
implementation, looking for a generic `yacc' program, will follow
in the future.