cmake fails with CMAKE_CXX_FLAGS="-Wall -Werror" - Redmine #2073

Using -Wall together with -Werror makes cmake fail:

cmake -DCMAKE_CXX_FLAGS="-Wall -Werror" ..
...
-- Performing Test CXX11_SUPPORTED - Failed
CMake Error at cmake/gmxTestCXX11.cmake:113 (message):
  This version of GROMACS requires a C++11 compiler.  Please use a newer
  compiler or use the GROMACS 5.1.x release.  See the installation guide for
  details.
Call Stack (most recent call first):
  CMakeLists.txt:164 (gmx_test_cxx11)


-- Configuring incomplete, errors occurred!

due to

build/CMakeFiles/CMakeTmp/src.cxx:45:11: error: unused variable ‘x’ [-Werror=unused-variable]
   double *x = nullptr;
           ^
cc1plus: all warnings being treated as errors

(from redmine: issue id 2073, created on 2016-11-13 by junghans, closed on 2017-01-20)

• Changesets:
  • Revision 31f0c056 by Aleksei Iupinov on 2017-01-05T14:33:29Z:

Fix compilation with CMAKE_CXX_FLAGS="-Wall -Werror"

Refs #2073

Change-Id: I9223b13e8abd251d56378052ace4fb4e9169a2aa