make all signatures go into a single file that's at the top

of the build directory.  This makes it so that scons won't
leave the .sconsign turds all over the source tree.

--HG--
extra : convert_revision : d530ed3eaed73c7ce2d21c7391f815051a37b500

build/SConstruct

@@ -239,6 +239,8 @@ default_env = Environment(ENV = os.environ,  # inherit user's enviroment vars
                           FULL_SYSTEM = False,
                           USE_MYSQL = False)
 
+default_env.SConsignFile("sconsign")
+
 # For some reason, the CC and CXX variables don't get passed into the
 # environment correctly.  This is probably some sort of scons bug that
 # will eventually be fixed.