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.