cmake DCHECK_HAS_OPEN_MPI examples

ginkgo-project/ginkgo CMakeLists.txt :409 

try_compile(
        gko_uses_openmpi
        ${Ginkgo_BINARY_DIR}
        ${Ginkgo_SOURCE_DIR}/cmake/openmpi_test.cpp
        COMPILE_DEFINITIONS -DCHECK_HAS_OPEN_MPI=1
        LINK_LIBRARIES MPI::MPI_CXX
    )