set proper shared library version

Index: wscript
--- wscript.orig
+++ wscript
@@ -186,7 +186,7 @@ def build(bld):
                 'includes':        ['include'],
                 'cflags':          ['-fvisibility=hidden'],
                 'lib':             ['m'],
-                'vnum':            SERD_VERSION,
+                'vnum':            os.environ['libserd_0_ltversion'],
                 'install_path':    '${LIBDIR}'}
     if bld.env.MSVC_COMPILER:
         lib_args['cflags'] = []