cmake: skip tests on mpich

tests/feature/CMakeLists.txt

@@ -26,7 +26,8 @@ try_compile(HAVE_OPENMPI
     LINK_LIBRARIES MPI::MPI_C
     OUTPUT_VARIABLE _try_out)
 if(NOT HAVE_OPENMPI)
-    message(FATAL_ERROR "DID NOT FIND OPENMPI\n\n${_try_out}")
+    message(WARNING "DID NOT FIND OPENMPI - skipping tests \n\n${_try_out}")
+    return()
 endif()
 include(FindUnixCommands)
 foreach( FEATURE