Update docs to recommend CMake >= v3.2.

[jlebar]

Summary

CMake v3.2 or newer is necessary to get interactive output when running
Lit via Ninja. Otherwise Ninja will buffer Lit's output, which makes
for a crummy experience -- you can't tell if your tests are hung!

[echristo]

Adding llvm-commits for the review.

[echristo]

LGTM.

Thanks!

-eric

[echristo]

Committed thusly:

dzur:~/sources/llvm> git svn dcommit
Committing to https://llvm.org/svn/llvm-project/llvm/trunk ...
M docs/CMake.rst
M docs/GettingStarted.rst
Committed r256791

docs/CMake.rst

 .. _Quick start:
 
 Quick start
 ===========
 
 We use here the command-line, non-interactive CMake interface.
 
 #. `Download <http://www.cmake.org/cmake/resources/software.html>`_ and install
-   CMake. Version 2.8.8 is the minimum required.
+   CMake. Version 2.8.8 is the minimum required, but if you're using the Ninja
+   backend, CMake v3.2 or newer is required to `get interactive output
+   <http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20141117/244797.html>`_
+   when running :doc:`Lit <CommandGuide/lit>`.
 
 #. Open a shell. Your development tools must be reachable from this shell
    through the PATH environment variable.
 
 #. Create a build directory. Building LLVM in the source
    directory is not supported. cd to this directory:
 
    .. code-block:: console

docs/GettingStarted.rst

#. Get the Test Suite Source Code **[Optional]**
    * ``cd where-you-want-llvm-to-live``
    * ``cd llvm/projects``
    * ``svn co http://llvm.org/svn/llvm-project/test-suite/trunk test-suite``
 
 #. Configure and build LLVM and Clang:
 
    The usual build uses `CMake <CMake.html>`_. If you would rather use
    autotools, see `Building LLVM with autotools <BuildingLLVMWithAutotools.html>`_.
+   Although the build is known to work with CMake >= 2.8.8, we recommend CMake
+   >= v3.2, especially if you're generating Ninja build files.
 
    * ``cd where you want to build llvm``
    * ``mkdir build``
    * ``cd build``
    * ``cmake -G <generator> [options] <path to llvm sources>``
 
      Some common generators are: