Adding check for broken version of Clang with Xcode 4.5.x.

- Also updating EXECUTE_PROCESS() calls to compiler to handle case
  when a compilation wrapper (e.g. ccache) is used.

Change-Id: I32837233a06ed1843125fb04852f82a9b8503769

docs/source/building.rst

diff --git a/docs/source/building.rst b/docs/source/building.rst
index 38a539b..49fdb3c 100644
--- a/docs/source/building.rst
+++ b/docs/source/building.rst
@@ -192,6 +192,12 @@
 <https://developer.apple.com/library/mac/#documentation/Performance/Conceptual/vecLib/Reference/reference.html>`_
 framework.
 
+.. NOTE::
+
+ Ceres will not compile using Xcode 4.5.x (Clang version 4.1) due to a bug in that version of
+ Clang.  If you are running Xcode 4.5.x, please update to Xcode >= 4.6.x before attempting to
+ build Ceres.
+
 .. code-block:: bash
 
       # CMake