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).

tests/functional_tests/CMakeLists.txt

@@ -73,8 +73,6 @@ target_sources(
 ###############################################################################
 if (BUILD_TESTS_ONLY)
   #TODO check that build_test_only still works with external-mpi
-  #find_package(MPI REQUIRED)
-  #find_package(hip REQUIRED PATHS /opt/rocm)
   find_package(rocshmem REQUIRED PATHS /opt/rocm)
 
   target_include_directories(
@@ -97,5 +95,5 @@ target_link_libraries(
   ${PROJECT_NAME}
   PRIVATE
     roc::rocshmem
-    PMIx::pmix
+    $<$<BOOL:${PMIx_FOUND}>:PMIx::pmix>
 )