[CMake] How to find vcvarsall.bat (e.g. at "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC")? CMAKE_LINKER?

[Niels Dekker - address until 2018]

We need to find a file named "vcvarsall.bat", located in the 
installation directory of Visual C++, which may have one of the 
following path names, for example:

   "C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\"
   "M:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\"
   "C:\Program Files\Microsoft Visual Studio 9.0\VC\"
   etc.

How can we retrieve this directory path name within our CMakeLists.txt? 
There's no standard CMake variable like ${CMAKE_COMPILER_DIR}, right?

Looking at a CMakeCache.txt file generated for Visual C++ 12 (2013) by 
CMake 2.8.12.2, I saw exactly one variable that would help us:

   CMAKE_LINKER:FILEPATH=C:/Program Files (x86)/Microsoft Visual Studio 
12.0/VC/bin/link.exe

 From there, we can easily find the path to the corresponding 
"vcvarsall.bat". However, ${CMAKE_LINKER} appears to be left 
undocumented intentionally, as it appears to be an internal variable:

   0008065: CMAKE_LINKER not mentioned on --help-variables
   http://www.cmake.org/Bug/view.php?id=8065
   Closed as "won't fix"

Does that mean we should not use ${CMAKE_LINKER}? If so, do you have 
another suggestion on how to find "vcvarsall.bat"?

Kind regards, Niels
-- 
Niels Dekker
Scientific programmer
LKEB, Leiden University Medical Center