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>.

diff --git a/configure.ac b/configure.ac
index f7319a153d52f6c6f6bb53fe7fd98fd5684f75c9..992e2033994c54ce40d4a9fcb9cd08cd2d5e83be 100644 (file)
--- a/configure.ac
+++ b/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