Unbundle SCons distribution.

Replace ./scons/scons.py with a wrapper script that starts the installed
version of scons. In this way, any packages relying on the bundled version
don't immediately break. This might be removed in the future.
Moved addition of version number to shared libraries from src/SConscript
to SConstruct, where it should be.  This is important for use with
extensions which require the Hall A libraries (like hcana, for example).