CMake: Add -T option to choose a generator toolset

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_TOOLSET and associated variable documentation to
hold the value persistently.

Add a RunCMake.GeneratorToolset test to cover basic "-T" option cases.
Verify that CMAKE_GENERATOR_TOOLSET is empty without -T, that -T is
rejected when the generator doesn't support it, and that two -T options
are always rejected.
VS: Implement generator toolset selection (#10722, #13774)

Implement generator toolset selection (cmake -T) for VS >= 10 by setting
the PlatformToolset.  Extend the RunCMake.GeneratorToolset test case to
verify CMAKE_GENERATOR_TOOLSET when the generator supports -T.

Since commit 485a940e (VS: Simplify MSVC version reporting, 2012-08-23)
all MSVC version information is detected during the compiler id step
from the actual compiler invoked by generated build systems rather than
hard-coded in VS generators.  Therefore we can set the PlatformToolset
in VS >= 10 project files and support toolsets from other VS versions.
Xcode: Implement generator toolset selection (#9831, #13802)

Implement generator toolset selection (cmake -T) for Xcode > 2.0 by
adding the GCC_VERSION build setting to project files.
CTest: Add options to set generator toolset

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_TOOLSET for -T.

The "ctest --build-and-test" mode already has "--build-generator" to
specify the -G option to CMake.  Add a "--build-generator-toolset" option
to specify the -T value.
ExternalProject: Propagate the generator toolset

When the CMAKE_GENERATOR option is given to ExternalProject_Add, look
also for option CMAKE_GENERATOR_TOOLSET to select the value of the cmake
"-T" command-line flag.  When no CMAKE_GENERATOR option is given
explicitly then use the current project's CMAKE_GENERATOR_TOOLSET (since
we already use its CMAKE_GENERATOR).
Merge branch 'master' into generator-toolset

We need the latest Tests/CMakeLists.txt so we can refactor all tests.
Tests: Consolidate ctest --build-and-test generator options

All ctest --build-and-test invocations require the options

  --build-generator ${CMAKE_GENERATOR}
  --build-makeprogram ${CMAKE_TEST_MAKEPROGRAM}

to be passed and have consistent values, except for a few special cases.
Collect the generator options in a variable instead of repeating the
options everywhere explicitly.
Tests: Add generator toolset support

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

56ca8d4 Tests: Add generator toolset support
f36c665 Tests: Consolidate ctest --build-and-test generator options
c0debb1 Merge branch 'master' into generator-toolset
daae0d2 ExternalProject: Propagate the generator toolset
e3841cf CTest: Add options to set generator toolset
f980a80 Xcode: Implement generator toolset selection (#9831, #13802)
650c647 VS: Implement generator toolset selection (#10722, #13774)
4fd5342 CMake: Add -T option to choose a generator toolset