fix cmake underlinkage error

fix cmake underlinkage error

The underlinkage MPI check under Ubuntu fails if MPI_CXX_LIBRARIES is
empty, which happens if you configure with CXX=mpicxx (even though you
should not).

In this situation, just disable this hack. Errors look like this:

CMake Error at cmake/modules/FindMPI.cmake:143 (LIST):
  LIST GET given empty list
Call Stack (most recent call first):
  cmake/macros/macro_find_package.cmake:27 (_FIND_PACKAGE)
  cmake/configure/configure_1_mpi.cmake:28 (FIND_PACKAGE)
/a/CMakeFiles/CMakeTmp/evaluate_expression.tmp:1
(FEATURE_MPI_FIND_EXTERNAL)
  cmake/macros/macro_evaluate_expression.cmake:30 (INCLUDE)
  cmake/macros/macro_configure_feature.cmake:237 (EVALUATE_EXPRESSION)
  cmake/configure/configure_1_mpi.cmake:81 (CONFIGURE_FEATURE)
  cmake/macros/macro_verbose_include.cmake:19 (INCLUDE)
  CMakeLists.txt:121 (VERBOSE_INCLUDE)

CMake Error at cmake/modules/FindMPI.cmake:144 (GET_FILENAME_COMPONENT):
  GET_FILENAME_COMPONENT called with incorrect number of arguments
Call Stack (most recent call first):
  cmake/macros/macro_find_package.cmake:27 (_FIND_PACKAGE)
  cmake/configure/configure_1_mpi.cmake:28 (FIND_PACKAGE)
/a/CMakeFiles/CMakeTmp/evaluate_expression.tmp:1
(FEATURE_MPI_FIND_EXTERNAL)
  cmake/macros/macro_evaluate_expression.cmake:30 (INCLUDE)
  cmake/macros/macro_configure_feature.cmake:237 (EVALUATE_EXPRESSION)
  cmake/configure/configure_1_mpi.cmake:81 (CONFIGURE_FEATURE)
  cmake/macros/macro_verbose_include.cmake:19 (INCLUDE)
  CMakeLists.txt:121 (VERBOSE_INCLUDE)

-- _mpi_libopen_pal_library not found! The call was:
--     FIND_LIBRARY(_mpi_libopen_pal_library NAMES open-pal HINTS
NO_DEFAULT_PATH NO_CMAKE_ENVIRONMENT_PATH NO_CMAKE_PATH
NO_SYSTEM_ENVIRONMENT_PATH NO_CMAKE_SYSTEM_PATH NO_CMAKE_FIND_ROOT_PATH)