set proper shared library version

Index: wscript
--- wscript.orig
+++ wscript
@@ -2,6 +2,7 @@
 
 from waflib import Build, Logs, Options
 from waflib.extras import autowaf
+import os
 
 # Library and package version (UNIX style major, minor, micro)
 # major increment <=> incompatible changes
@@ -120,7 +121,7 @@ def build(bld):
             uselib          = 'SERD SORD LV2',
             name            = 'libsratom',
             target          = 'sratom-%s' % SRATOM_MAJOR_VERSION,
-            vnum            = SRATOM_VERSION,
+            vnum            = os.environ['libsratom_0_ltversion'],
             install_path    = '${LIBDIR}',
             defines         = defines + ['SRATOM_INTERNAL'],
             cflags          = libflags)