VS: Save system name and version in global generator members

Add to cmGlobalVisualStudio10Generator members for SystemName and
SystemVersion and populate them in SetSystemName.
VS: Add a hook to adapt to SystemName and SystemVersion

Add a virtual cmGlobalVisualStudio10Generator::InitializeSystem method
called from SetSystemName once the SystemName and SystemVersion members
have been populated.  This will give VS version-specific generators a
chance to recognize and adapt to the target system.
Merge topic 'vs10-system-hook' into next

e58f9753 VS: Add a hook to adapt to SystemName and SystemVersion
d0dd28fa VS: Save system name and version in global generator members