Multiverso/Test not using cmake variables for MPI compiler

[jbalma]

The build of Multiverso/Test files have hard-coded "mpicxx" compiler instead of respecting the option
cmake -D MPI_CXX_COMPILER=CC, which breaks builds in Cray Linux environments, and probably any user environment where compiler wrappers are used. This needs to be fixed to support building in environments where MPI compiler wrappers are common (i.e. CC, cc, ftn, etc).

For building on Cray XC systems I'm configuring multiverso with cmake/3.5.2 as follows:

cmake -D MPI_CXX_COMPILER=CC -D MPI_C_COMPILER=cc -D CMAKE_LINKER=CC -D CMAKE_CXX_COMPILER=CC -D CMAKE_C_COMPILER=cc -D CMAKE_SYSTEM_NAME=CrayLinuxEnvironment -D CMAKE_BUILD_TYPE="release" -D BUILD_SHARED_LIBS=ON -D TEST=OFF -D BOOST_ROOT=/path/to/Boost/boost-1.60.0 -D BOOST_INCLUDEDIR=/path/to/Boost/boost-1.60.0/include -D MPI_LIBRARY=${CRAY_MPICH_DIR}/lib/libmpich.so -D MPI_EXTRA_LIBRARY=${CRAY_MPICH_DIR}/lib -D CMAKE_VERBOSE_MAKEFILE=TRUE -D MPI_CXX_INCLUDE_PATH=${CRAY_MPICH_DIR}/include -D MPIEXEC=/path/to/srun -D MPIEXEC_NUMPROC_FLAG="-n" ../

-Jake

[jbalma]

Same issue with the "CMakeFiles/multiverso.test.dir/link.txt" file