title: Building Eigen3 with CXX_FLAGS
body: <p>I'm trying to build Eigen3 using CMake as described in the installation menual:</p>
  
  <pre><code>root@60e449767588:/opt/tr/src/eigen/build_dir$ ../../cmake-3.8.1/bin/cmake  -DCMAKE_BUILD_TYPE=Release --build ..
  </code></pre>
  
  <p>This is the output:</p>
  
  <pre><code>-- Standard libraries to link to explicitly: none
  -- Found unsuitable Qt version "" from NOTFOUND
  -- Could NOT find CHOLMOD (missing:  CHOLMOD_INCLUDES CHOLMOD_LIBRARIES) 
  -- Could NOT find UMFPACK (missing:  UMFPACK_INCLUDES UMFPACK_LIBRARIES) 
  -- Could NOT find SUPERLU (missing:  SUPERLU_INCLUDES SUPERLU_LIBRARIES SUPERLU_VERSION_OK) 
  -- Could NOT find PASTIX (missing:  PASTIX_INCLUDES PASTIX_LIBRARIES) 
  -- Could NOT find SCOTCH (missing:  SCOTCH_INCLUDES SCOTCH_LIBRARIES) 
  -- Could NOT find METIS (missing:  METIS_INCLUDES METIS_LIBRARIES) 
  -- Could NOT find SPQR (missing:  SPQR_INCLUDES SPQR_LIBRARIES) 
  -- Found unsuitable Qt version "" from NOTFOUND
  -- Boost version: 1.64.0
  -- Could NOT find GOOGLEHASH (missing:  GOOGLEHASH_INCLUDES GOOGLEHASH_COMPILE) 
  -- Could NOT find ADOLC (missing:  ADOLC_INCLUDES ADOLC_LIBRARIES) 
  -- Could NOT find MPFR (missing:  MPFR_INCLUDES MPFR_LIBRARIES MPFR_VERSION_OK) (Required is at least version "2.3.0")
  -- Could NOT find GMP (missing:  GMP_INCLUDES GMP_LIBRARIES) 
  -- Could NOT find PkgConfig (missing:  PKG_CONFIG_EXECUTABLE) 
  -- Could NOT find FFTW (missing:  FFTW_INCLUDES FFTW_LIBRARIES) 
  -- Could NOT find OpenGL (missing:  OPENGL_gl_LIBRARY OPENGL_INCLUDE_DIR) 
  -- Could NOT find GLUT (missing:  GLUT_glut_LIBRARY GLUT_INCLUDE_DIR) 
  CUDA_TOOLKIT_ROOT_DIR not found or specified
  -- Could NOT find CUDA (missing:  CUDA_TOOLKIT_ROOT_DIR CUDA_NVCC_EXECUTABLE CUDA_INCLUDE_DIRS CUDA_CUDART_LIBRARY) (Required is at least version "7.0")
  -- Found unsuitable Qt version "" from NOTFOUND
  -- Qt4 not found, so disabling the mandelbrot and opengl demos
  -- Could NOT find CHOLMOD (missing:  CHOLMOD_INCLUDES CHOLMOD_LIBRARIES) 
  -- Could NOT find UMFPACK (missing:  UMFPACK_INCLUDES UMFPACK_LIBRARIES) 
  -- Could NOT find SUPERLU (missing:  SUPERLU_INCLUDES SUPERLU_LIBRARIES SUPERLU_VERSION_OK) 
  -- Could NOT find PASTIX (missing:  PASTIX_INCLUDES PASTIX_LIBRARIES) 
  -- Could NOT find SCOTCH (missing:  SCOTCH_INCLUDES SCOTCH_LIBRARIES) 
  -- Could NOT find METIS (missing:  METIS_INCLUDES METIS_LIBRARIES) 
  -- ************************************************************
  -- ***    Eigen's unit tests configuration summary          ***
  -- ************************************************************
  -- 
  -- Build type:        Release
  -- Build site:        60e449767588
  -- Build string:      linux-4.4.0-78-generic-_-4.9.2-sse2-64bit
  -- Enabled backends:  Boost.Multiprecision, 
  -- Disabled backends: Cholmod,  UmfPack,  SuperLU,  PaStiX,  METIS,  SPQR,  Qt4 support,  GoogleHash,  Adolc,  MPFR C++,  fftw,  OpenGL, 
  -- Default order:     Column-major
  -- Maximal matrix/vector size: 320
  -- SSE2:              Using architecture defaults
  -- SSE3:              Using architecture defaults
  -- SSSE3:             Using architecture defaults
  -- SSE4.1:            Using architecture defaults
  -- SSE4.2:            Using architecture defaults
  -- AVX:               Using architecture defaults
  -- FMA:               Using architecture defaults
  -- AVX512:            Using architecture defaults
  -- Altivec:           Using architecture defaults
  -- VSX:               Using architecture defaults
  -- ARM NEON:          Using architecture defaults
  -- ARMv8 NEON:        Using architecture defaults
  -- S390X ZVECTOR:     Using architecture defaults
  -- C++11:             OFF
  -- SYCL:              OFF
  -- CUDA:              OFF
  -- 
  CXX:               /usr/bin/c++
   CXX_VERSION:       c++ (Debian 4.9.2-10) 4.9.2
   CXX_FLAGS:         -O3 -DNDEBUG -fPIC -DEIGEN_USE_MKL_ALL -DMKL_ILP64 -fopenmp -m64 -v -pedantic -Wall -Wextra -Wundef -Wcast-align -Wchar-subscripts -Wnon-virtual-dtor -Wunused-local-typedefs -Wpointer-arith -Wwrite-strings -Wformat-security -Wlogical-op -Wdouble-promotion -Wno-psabi -Wno-variadic-macros -Wno-long-long -fno-check-new -fno-common -fstrict-aliasing -ansi
   Sparse lib flags:   
  
  -- ************************************************************
  -- 
  -- Configured Eigen 3.3.3
  -- 
  -- Some things you can do now:
  -- --------------+--------------------------------------------------------------
  -- Command       |   Description
  -- --------------+--------------------------------------------------------------
  -- make install  | Install Eigen. Headers will be installed to:
  --               |     &lt;CMAKE_INSTALL_PREFIX&gt;/&lt;INCLUDE_INSTALL_DIR&gt;
  --               |   Using the following values:
  --               |     CMAKE_INSTALL_PREFIX: /usr/local
  --               |     INCLUDE_INSTALL_DIR:  include/eigen3
  --               |   Change the install location of Eigen headers using:
  --               |     cmake . -DCMAKE_INSTALL_PREFIX=yourprefix
  --               |   Or:
  --               |     cmake . -DINCLUDE_INSTALL_DIR=yourdir
  -- make doc      | Generate the API documentation, requires Doxygen &amp; LaTeX
  -- make check    | Build and run the unit-tests. Read this page:
  --               |   http://eigen.tuxfamily.org/index.php?title=Tests
  -- make blas     | Build BLAS library (not the same thing as Eigen)
  -- make uninstall| Removes files installed by make install
  -- --------------+--------------------------------------------------------------
  -- 
  -- Configuring done
  -- Generating done
  -- Build files have been written to: /opt/tr/src/eigen/build_dir
  </code></pre>
  
  <p>My questions are:</p>
  
  <ol>
  <li><p>How to remove the default value of CXX_FLAGS?</p></li>
  <li><p>How to specify that I wish to work with C++11 or even 14?</p></li>
  <li><p>How to specify that I wish to work with SYCL?</p></li>
  </ol>
  
  <p>Thank you!</p>