configure.ac: bump to version 4.0.91 for the alpha release
diff --git a/configure.ac b/configure.ac
index 6f9ad43..093d6b5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,6 +1,6 @@
 m4_define([weston_major_version], [4])
 m4_define([weston_minor_version], [0])
-m4_define([weston_micro_version], [90])
+m4_define([weston_micro_version], [91])
 m4_define([weston_version],
           [weston_major_version.weston_minor_version.weston_micro_version])
 m4_define([libweston_major_version], [5])