VS: Add CMAKE_VS_(DEVENV|MSBUILD|MSDEV)_COMMAND variables

Since commit 5f5c92b9 (VS: Add internal APIs to find MSBuild,
devenv/VCExpress, and msdev, 2013-11-13) the VS generators have
known how to lookup the locations of their build tools directly.
Expose this information to CMake language code by defining new
variables to hold the paths to these tools.
CMakeDetermineCompilerId: Use CMAKE_VS_(DEVENV|MSBUILD|MSDEV)_COMMAND

In the code path for launching the VS IDE tools, avoid using
CMAKE_MAKE_PROGRAM.  Instead use the variables CMAKE_VS_DEVENV_COMMAND,
CMAKE_VS_MSBUILD_COMMAND, and CMAKE_VS_MSDEV_COMMAND to lookup the
location of the build tool needed.  Choose the proper tool based on
availability and necessity for the language (e.g. Intel Fortran must
build with devenv.com and not MSBuild.exe).
Merge topic 'cleanup-build-commands'

e420124 CMakeDetermineCompilerId: Use CMAKE_VS_(DEVENV|MSBUILD|MSDEV)_COMMAND
0c55729 VS: Add CMAKE_VS_(DEVENV|MSBUILD|MSDEV)_COMMAND variables