OpenMPI 4.0.3 forgets to link open-pal

Hi,

I am trying to compile a C++ program with CMake 3.16.3 on Ubuntu 20.04 LTS, which ships OpenMPI 4.0.3.

I realized that the
  mpic++ --showme:link
flags forget to link `-lopen-pal`, which is part of the OpenMPI lib dir.
  -pthread -L/usr/lib/x86_64-linux-gnu/openmpi/lib -lmpi_cxx -lmpi

Following that, a CMake CXX program compiled with clang++ that checks for `find_package(MPI REQUIRED)` will error out with:
```
-- Found MPI_C: /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi.so (found version "3.1")
-- Could NOT find MPI_CXX (missing: MPI_CXX_WORKS)
CMake Error at /usr/local/share/cmake-3.21/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
-- Configuring incomplete, errors occurred!
  Could NOT find MPI (missing: MPI_CXX_FOUND CXX) (found version "3.1")
```

The reason for that is in the compile check
```
/opt/rocm/llvm/bin/clang++ -pthread CMakeFiles/cmTC_abf94.dir/test_mpi.cpp.o -o cmTC_abf94 -Wl,-rpath,/usr/lib/x86_64-linux-gnu/openmpi/lib /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi.so
ld.lld: error: /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: undefined reference to opal_class_init_epoch [--no-allow-shlib-undefined]
ld.lld: error: /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: undefined reference to opal_list_item_t_class [--no-allow-shlib-undefined]
ld.lld: error: /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: undefined reference to opal_class_initialize [--no-allow-shlib-undefined]
ld.lld: error: /usr/lib/x86_64-linux-gnu/openmpi/lib/libmpi_cxx.so: undefined reference to opal_uses_threads [--no-allow-shlib-undefined]
```

where the linker refuses the missing lib and fails the check.

The solution to this is to make sure that
  mpic++ --showme:link
appends
  -lopen-pal
as well.

User-side work-around until fix is applied:
  export LDFLAGS="-lopen-pal"