configure.ac: bump to version 5.0.0 for the official release

configure.ac

-m4_define([weston_major_version], [4])
+m4_define([weston_major_version], [5])
 m4_define([weston_minor_version], [0])
-m4_define([weston_micro_version], [94])
+m4_define([weston_micro_version], [0])
 m4_define([weston_version],
           [weston_major_version.weston_minor_version.weston_micro_version])
 m4_define([libweston_major_version], [5])