There is not much point in compatibility with various lex incarnations,
since the generated files are distributed and the user does not need
any lex tool to simply build from a tarball.
This additionally avoids a problem with AC_PROG_LEX which can hard-fail
the configure run when the selected lex command is not working, even
if the tool is not needed for the build. Now, a fatal error will only
be generated if the tool is required at make time.