set proper shared library version

Index: wscript
--- wscript.orig
+++ wscript
@@ -123,7 +123,7 @@ def build(bld):
                   export_includes = ['.'],
                   name            = 'libsord',
                   target          = 'sord-%s' % SORD_MAJOR_VERSION,
-                  vnum            = SORD_VERSION,
+                  vnum            = os.environ['libsord_0_ltversion'],
                   install_path    = '${LIBDIR}',
                   libs            = libs,
                   uselib          = 'SERD',