Update docs to recommend CMake >= v3.2.

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!

Patch by Justin Lebar!

llvm-svn: 256791

docs/CMake.rst

@@ -26,7 +26,10 @@ 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.

docs/GettingStarted.rst

@@ -78,6 +78,8 @@ Here's the short story for getting up and running quickly with LLVM:
 
    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``