cmGlobalGenerator: Call SetGeneratorToolset even for empty toolset

Move handling of an empty toolset name into the implementation of the
method.  This simplifies the VS 10 implementation of default toolset
selection because it has one code path that is always called.
cmGlobalVisualStudio10Generator: Re-order some methods

Order SetSystemName and SetGeneratorToolset method declarations
and definitions as they are called.
VS: Refactor internal default platform name selection

Rename the 'PlatformName' member to 'DefaultPlatformName' and make
sure it is only read through a 'GetPlatformName()' call.  This will
allow non-default names to be chosen later.
Help: Document CMAKE_VS_PLATFORM_NAME variable
CMake: Add CMAKE_GENERATOR_PLATFORM option

Reject the option by default.  It will be implemented on a per-generator
basis.  Pass the setting into try_compile project generation.  Add cache
entry CMAKE_GENERATOR_PLATFORM and associated variable documentation to
hold the value persistently.

Add a RunCMake.GeneratorPlatform test to cover basic use cases for the
option.  Verify that CMAKE_GENERATOR_PLATFORM is empty by default, and
that it is rejected when the generator does not support a user setting.
VS: Implement CMAKE_GENERATOR_PLATFORM for VS >= 8

For VS generator names that do not specify the platform name, read
CMAKE_GENERATOR_PLATFORM to get it.

Extend the RunCMake.GeneratorPlatform test with a case covering
use of the x64 platform when the test generator is a Visual Studio
generator whose name does not specify a platform.
CTest: Add options to set generator platform

The ctest_configure command already reads the CTEST_CMAKE_GENERATOR
variable to get the value for the cmake -G option.  Read new variable
CTEST_CMAKE_GENERATOR_PLATFORM to pass on as CMAKE_GENERATOR_PLATFORM.

The "ctest --build-and-test" mode already has "--build-generator" to
specify the -G option to CMake.  Add a "--build-generator-platform" option
to specify a value to pass on as CMAKE_GENERATOR_PLATFORM.
ExternalProject: Propagate the generator platform

When the CMAKE_GENERATOR option is given to ExternalProject_Add, look
also for option CMAKE_GENERATOR_PLATFORM to pass on to cmake as a cache
definition.  When no CMAKE_GENERATOR option is given explicitly then use
the current project's CMAKE_GENERATOR_PLATFORM (since we already use its
CMAKE_GENERATOR).
Tests: Add generator platform support

Propagate CMAKE_GENERATOR_PLATFORM through the test hierarchy so that all
tests can build with the selected generator platform, if any.
Merge topic 'vs-generator-platform' into next

e17751da Tests: Add generator platform support
6944997b ExternalProject: Propagate the generator platform
8d332091 CTest: Add options to set generator platform
b97736a2 VS: Implement CMAKE_GENERATOR_PLATFORM for VS >= 8
0f1f1271 CMake: Add CMAKE_GENERATOR_PLATFORM option
4f7d0c42 Help: Document CMAKE_VS_PLATFORM_NAME variable
68d4280a VS: Refactor internal default platform name selection
ad2a4776 cmGlobalVisualStudio10Generator: Re-order some methods
03b7b6cd cmGlobalGenerator: Call SetGeneratorToolset even for empty toolset