Remove MPI compile-time dependency (#264)

* use dlsym for MPI functions

to allow compiling without MPI support, convert the usage of MPI functions and symbols to be based on a dlopen/dlsym based mechanism. Turns out this cannot be done entirely vendor neutral, slightly different solutions might be required for Open MPI, MPICH and the new MPI ABI.

* checkpoint

more work to be done.

* checkpoint 2

* checkpoint 3

* checkpoint 4

examples compile and link correctly

* checkpoitn 5 (I think)

* Checkpoitn 6

* dyld-mpi: adapt GDA

* dyldmpi: tests that depend on MPI need to link with it themselves

* do not ../mpi_instance.h

* dyldmpi: make the symetricHeapTestFixture compile

* dyldmpi: Change cmakery, compiles and run gda w/o external MPI

* Make it also compile in external MPI mode

* dyldmpi: ipc unit tests compile but do not link

* dyldmpi: new approach, if external mpi required, link with mpi,
otherwise use ompi5 abi

* C-style comments in cmakelist..

* dyldmpi: examples: do not fail compiling if MPI not found at build time,
instead do not compile the MPI required examples

* more updates to CMake logic

* convert RO backend

and a few other cleanups

* update some unit tests

to work with the dlopen MPI environment correctly.

---------

Co-authored-by: Aurelien Bouteiller <abouteil@amd.com>

[ROCm/rocshmem commit: e4c427a736]

projects/rocshmem/cmake/rocshmem_config.h.in

@@ -45,3 +45,5 @@
 #cmakedefine GDA_IONIC
 #cmakedefine GDA_BNXT
 #cmakedefine GDA_MLX5
+#cmakedefine USE_EXTERNAL_MPI
+#cmakedefine HAVE_EXTERNAL_MPI