Do not update CMAKE_REQUIRED_FLAGS to match CMAKE_CXX_FLAGS.

- This was a hangover from the pre-Ceres 2.0 era when we needed to
  ensure that -std=c++11 was passed to check_cxx_source_compiles() in
  FindUnorderedMap/FindSharedPtr if the user had enabled CXX11.
- Unfortunately, the same variable (CMAKE_REQUIRED_FLAGS) is used for
  both check_cxx_source_compiles() and check_c_source_compiles(), and
  thus updating it to include -std=c++11 would thus break any checks
  for C, which are made internally in the version of FindThreads.cmake
  shipped with CMake which could cause build breakage as identified
  in issue #355.

Change-Id: I36be1a21db14e3839eaf955a80bd952ef40fa269

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index efbce67..b02bf7a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -435,11 +435,6 @@
   message("-- Building without OpenMP, disabling.")
 endif (OPENMP)
 
-# Initialise CMAKE_REQUIRED_FLAGS used by CheckCXXSourceCompiles with the
-# contents of CMAKE_CXX_FLAGS such that if the user has passed extra flags
-# they are used when checking for compiler features.
-set(CMAKE_REQUIRED_FLAGS ${CMAKE_CXX_FLAGS})
-
 if (TBB)
   find_package(TBB QUIET)
   if (TBB_FOUND)