Change library numbering scheme

Fixes https://github.com/harfbuzz/harfbuzz/issues/1431
diff --git a/configure.ac b/configure.ac
index abe86ed..14edcc3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -45,7 +45,7 @@
 
 # Libtool version
 m4_define([hb_version_int],
-	  m4_eval(hb_version_major*10000 + hb_version_minor*100 + hb_version_micro))
+	  m4_eval(60000 + hb_version_major*100 + hb_version_minor*10 + hb_version_micro))
 HB_LIBTOOL_VERSION_INFO=hb_version_int:0:hb_version_int
 AC_SUBST(HB_LIBTOOL_VERSION_INFO)