build: don't require flex for ordinary builds

* configure.ac (LEX): Don't fail if this is lex, as flex is not
required for ordinary builds.  Instead, issue a warning and
substitute a no-op LEX.  Reported by Michael Felt in
<http://lists.gnu.org/archive/html/bug-bison/2013-08/msg00009.html>.

configure.ac

@@ -175,7 +175,10 @@ AC_SUBST([YACC_LIBRARY])
 # Checks for programs.
 AM_MISSING_PROG([DOT], [dot])
 AC_PROG_LEX
-$LEX_IS_FLEX || AC_MSG_ERROR([Flex is required])
+$LEX_IS_FLEX || test "X$LEX" = X: || {
+  AC_MSG_WARN([bypassing lex because flex is required])
+  LEX=:
+}
 AC_PROG_YACC
 AC_PROG_RANLIB
 AC_PROG_GNU_M4