Tell generators about CMAKE_SYSTEM_NAME as soon as it is known

Add a cmGlobalGenerator::SetSystemName virtual method.  Call it from
cmGlobalGenerator::EnableLanguage as soon as CMAKE_SYSTEM_NAME is known.
This will give generators a chance to adapt themselves to the target
system.  Pass the cmMakefile instance to the method so that the
generator can update it accordingly.  Return early from EnableLanguage
if SetSystemName returns false.
VS: Refactor CMAKE_FORCE_*64 platform definitions

Remove the general infrastructure for these additional platform
definitions and hard-code the only two special cases that used
it.  They are only for historical reasons so no new such cases
should be added.
VS: Remove unused method from VS 9 generator
VS: Delay platform definitions until system name is known

Move the definition of CMAKE_VS_PLATFORM_NAME and other variables that
are not needed by CMakeDetermineSystem out of the AddPlatformDefinitions
method and into a SetSystemName method.  The latter may later use
CMAKE_SYSTEM_NAME to decide what platform-specific definitions to add.
VS: Delay detection of VS 10 Express 64-bit tools

Wait until SetSystemName when the CMAKE_SYSTEM_NAME is known.
Later the decision to use 64-bit tools may depend on it.
Merge topic 'delay-generator-platform' into next

e7fdb44b VS: Delay detection of VS 10 Express 64-bit tools
6e176e6d VS: Delay platform definitions until system name is known
d3d9218a VS: Remove unused method from VS 9 generator
eeb60102 VS: Refactor CMAKE_FORCE_*64 platform definitions
ecb34faa Tell generators about CMAKE_SYSTEM_NAME as soon as it is known