xen: Added support to build agent in xen builder

- Added platform detection setting to selectively turn off Wno-unused-local-typedefs
 for gcc 4.1.2
- Reverted changed in GetBuildInfo() as GCC 4.1.2 complained with warning,
  'type always evaluates to true'
- Fix in agent_param.cc, ~AgentParam() [Praveen]
- Fix in return data type CalculateBandwidth() - Fix in data type comparison in test_vgw.cc

Signed-off-by: Anirban Chakraborty <abchak@juniper.net>