configure: look for a lex program to be used by the testsuite

configure: look for a lex program to be used by the testsuite

This will allow the testcases requiring a 'lex' program to run also
with vendor/legacy lex implementations, not only with 'flex'.

* configure.ac: Look for a lex program, using AC_CHECK_PROGS.
* tests/defs.in: New required entry 'lex'.
($LEX): Let the user override the lex program to be used by the
testsuite.
* tests/cond35.test ($required): Require 'lex', not 'flex'.
* tests/cond36.test: Likewise.
* tests/lexv3.test: Likewise.
* tests/lexv3.test: Likewise.
* tests/silent-lex-gcc.test: Likewise.
* tests/silent-lex-generic.test: Likewise.
* tests/silent-many-gcc.test: Likewise.
* tests/silent-many-generic.test:likewise.
* tests/lexvpath.test: Likewise, and fix typo in comments.