update to build system: (#277)

- make adding PMIx library to compile time based on the result of
  finding PMIx support. This is required eg if compiling rocSHMEM with ompi
  4.0/4.1, which do not have a built-in PMIx version.
- when setting USE_EXTERNAL_MPI=OFF  which ensures that we do not
  check for external MPI libraries (even if one would be available).

[ROCm/rocshmem commit: ed957302d4]

projects/rocshmem/examples/CMakeLists.txt

@@ -29,7 +29,6 @@ cmake_minimum_required(VERSION 3.16.3 FATAL_ERROR)
 include(${CMAKE_SOURCE_DIR}/cmake/setup_project.cmake)
 project(rocshmem_examples VERSION 1.0.0 LANGUAGES CXX)
 
-find_package(MPI)
 find_package(hip REQUIRED PATHS /opt/rocm)
 if (NOT TARGET roc::rocshmem)
   find_package(rocshmem REQUIRED PATHS /opt/rocm)