Index: configure.ac
===================================================================
--- configure.ac	(revision 183630)
+++ configure.ac	(working copy)
@@ -193,6 +193,25 @@
 fi
 AC_SUBST(DOWNLOAD)
 
+if test main/ast_expr2.y -nt main/ast_expr2.c ; then
+	if test -d main/.svn ; then
+		AC_PATH_PROG([WC], [wc], :)
+		AC_PATH_PROG([SVN], [svn], :)
+		AC_PATH_PROG([TOUCH], [touch], :)
+		if test `${SVN} diff main/ast_expr2.y | ${WC} -l` -gt 0 ; then
+			AC_PATH_PROG([BISON], [bison], :)
+			if test "${BISON}" = ":" ; then
+                  		AC_MSG_NOTICE([***])
+				AC_MSG_NOTICE([*** It appears that you do not have bison installed.])
+                  		AC_MSG_NOTICE([*** Please install it to regenerate main/ast_expr2.c, or revert the changes to main/ast_expr2.y])
+				exit 1
+			fi
+		else
+			${TOUCH} main/ast_expr2.c
+		fi
+	fi
+fi
+
 AC_CHECK_TOOL([SOXMIX], [soxmix], [:])
 if test "${SOXMIX}" != ":" ; then
 	AC_DEFINE([HAVE_SOXMIX], 1, [Define to 1 if your system has soxmix application.])