Also require C++14 when building gtest

[masterleinad]

While building with XL, I also needed to request C++14 for building gtest to make that work. I don't see any real drawbacks since we require C++14 anyway.

[dalg24]

While building with XL, I also needed to request C++14 for building gtest to make that work.

Please elaborate

[masterleinad]

Please elaborate

Without this patch, I get

/sw/summit/xl/16.1.1-9/xlC/16.1.1/bin/xlc++ -x c++ -DGTEST_HAS_PTHREAD=0 -DGTEST_HAS_TR1_TUPLE=0 -I/ccs/home/darndt/kokkos/tpls/gtest -I/ccs/home/darndt/kokkos/build/core/unit_test -I/ccs/home/darndt/kokkos/core/unit_test -F /ccs/home/darndt/kokkos/build/my_xl_configuration -std=c++14 -D__cplusplus=201402L -g -std=gnu++11 -o CMakeFiles/kokkos_gtest.dir/__/__/tpls/gtest/gtest/gtest-all.cc.o -c /ccs/home/darndt/kokkos/tpls/gtest/gtest/gtest-all.cc
In file included from <built-in>:395:
<command line>:8:9: warning: '__cplusplus' macro redefined [-Wmacro-redefined]
#define __cplusplus 201402L
        ^
<built-in>:358:9: note: previous definition is here
#define __cplusplus 201103L
        ^
In file included from /ccs/home/darndt/kokkos/tpls/gtest/gtest/gtest-all.cc:39:
In file included from /ccs/home/darndt/kokkos/tpls/gtest/gtest/gtest.h:3074:
/autofs/nccs-svm1_sw/summit/gcc/6.4.0/bin/../lib/gcc/powerpc64le-none-linux-gnu/6.4.0/../../../../include/c++/6.4.0/iomanip:460:12: error: 
      'auto' return without trailing return type; deduced return types are a C++14 extension
    inline auto
           ^
/autofs/nccs-svm1_sw/summit/gcc/6.4.0/bin/../lib/gcc/powerpc64le-none-linux-gnu/6.4.0/../../../../include/c++/6.4.0/iomanip:469:12: error: 
      'auto' return without trailing return type; deduced return types are a C++14 extension
    inline auto
           ^
/autofs/nccs-svm1_sw/summit/gcc/6.4.0/bin/../lib/gcc/powerpc64le-none-linux-gnu/6.4.0/../../../../include/c++/6.4.0/iomanip:479:12: error: 
      'auto' return without trailing return type; deduced return types are a C++14 extension
    inline auto
           ^
In file included from /ccs/home/darndt/kokkos/tpls/gtest/gtest/gtest-all.cc:316:
In file included from /autofs/nccs-svm1_sw/summit/gcc/6.4.0/bin/../lib/gcc/powerpc64le-none-linux-gnu/6.4.0/../../../../include/c++/6.4.0/algorithm:62:
/autofs/nccs-svm1_sw/summit/gcc/6.4.0/bin/../lib/gcc/powerpc64le-none-linux-gnu/6.4.0/../../../../include/c++/6.4.0/bits/stl_algo.h:3352:7: error: 
      statement not allowed in constexpr function
      while (__first != __last)
      ^
/autofs/nccs-svm1_sw/summit/gcc/6.4.0/bin/../lib/gcc/powerpc64le-none-linux-gnu/6.4.0/../../../../include/c++/6.4.0/bits/stl_algo.h:5459:7: error: 
      statement not allowed in constexpr function
      while (++__first != __last)
      ^
/autofs/nccs-svm1_sw/summit/gcc/6.4.0/bin/../lib/gcc/powerpc64le-none-linux-gnu/6.4.0/../../../../include/c++/6.4.0/bits/stl_algo.h:5523:7: error: 
      statement not allowed in constexpr function
      while (++__first != __last)
      ^
1 warning and 6 errors generated.
Error while processing /ccs/home/darndt/kokkos/tpls/gtest/gtest/gtest-all.cc.

Of course, you can argue that I do all kinds of horrible things by specifying CMAKE_CXX_FLAGS=-std=c++14 -D__cplusplus=201402L but that's the only way I was able to convince XL to compile Kokkos.