add preset to make it easier to use clang with OpenMP and MPI

# preset that will enable clang/clang++ with support for MPI and OpenMP (on Linux boxes)

set(CMAKE_CXX_COMPILER "clang++" CACHE STRING "" FORCE)

set(CMAKE_C_COMPILER "clang" CACHE STRING "" FORCE)

set(CMAKE_CXX_FLAGS "-Wall -Wextra -g -O2 -DNDEBG" CACHE STRING "" FORCE)

set(MPI_CXX "clang++" CACHE STRING "" FORCE)

set(MPI_CXX_COMPILER "mpicxx" CACHE STRING "" FORCE)

set(OpenMP_C "clang" CACHE STRING "" FORCE)

set(OpenMP_C_FLAGS "-fopenmp" CACHE STRING "" FORCE)

set(OpenMP_C_LIB_NAMES "omp" CACHE STRING "" FORCE)

set(OpenMP_CXX "clang++" CACHE STRING "" FORCE)

set(OpenMP_CXX_FLAGS "-fopenmp" CACHE STRING "" FORCE)

set(OpenMP_CXX_LIB_NAMES "omp" CACHE STRING "" FORCE)

set(OpenMP_omp_LIBRARY "/usr/lib64/libomp.so" CACHE PATH "" FORCE)