Update default abi-bits to 64 on 64-bit systems

The Waf configuration uses 32 ABI bits as the default even on 64-bit
Unix system, which typically do not have the appropriate libraries
installed to build 32-bit objects.  This changeset changes the default
configuration to use 64 ABI bits on 64-bit systems.
Fix syntax error on python 2.6
Add bde_gitcheckout.py and bde_gitutil.py
Parse command line arguments
Use python optparse
Make bde_repo.py executable