generate a soname that respects SHARED_LIBS

Index: SCons/Tool/linkCommon/SharedLibrary.py
--- SCons/Tool/linkCommon/SharedLibrary.py.orig
+++ SCons/Tool/linkCommon/SharedLibrary.py
@@ -93,9 +93,8 @@ def _soversion(target, source, env, for_signature):
     if "SOVERSION" in env:
         return ".$SOVERSION"
     elif "SHLIBVERSION" in env:
-        shlibversion = env.subst("$SHLIBVERSION")
-        # We use only the most significant digit of SHLIBVERSION
-        return "." + shlibversion.split(".")[0]
+        shlibversion = env.subst("$SHLIBVERSION").split(".")
+        return "." + shlibversion[0] + "." + shlibversion[1]
     else:
         return ""