set proper shared library version

Index: wscript
--- wscript.orig
+++ wscript
@@ -357,7 +357,7 @@ def build(bld):
                   includes        = ['.', 'include', './src'],
                   name            = 'liblilv',
                   target          = 'lilv-%s' % LILV_MAJOR_VERSION,
-                  vnum            = LILV_VERSION,
+                  vnum            = os.environ['liblilv_0_ltversion'],
                   install_path    = '${LIBDIR}',
                   defines         = ['LILV_INTERNAL', 'ZIX_STATIC'],
                   cflags          = libflags,