diff --git a/configure.ac b/configure.ac
index b21fd44800..94a92c7ce5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -159,8 +159,8 @@ NEED_PROG(patch, patch)
 AC_PATH_PROG(xmllint, xmllint, false)
 AC_PATH_PROG(xsltproc, xsltproc, false)
 AC_PATH_PROG(w3m, w3m, false)
-NEED_PROG(flex, flex)
-NEED_PROG(bison, bison)
+AC_PATH_PROG(flex, flex, false)
+AC_PATH_PROG(bison, bison, false)
 NEED_PROG(perl, perl)
 NEED_PROG(sed, sed)
 NEED_PROG(tar, tar)