Use find_package for MPI (#92)

* Use find_package for MPI
* Minor fixes

[ROCm/rccl-tests commit: 46152785f0]

projects/rccl-tests/README.md

@@ -30,9 +30,7 @@ $ make
 When using the cmake build procedure, please make sure that RCCL has also been built using cmake (i.e. not using the install.sh script), since cmake will check
 for cmake target and config files that are created during the RCCL build.
 
-Using the cmake method also has the advantage that the build is automatically checking for MPI installations, i.e. it is not necessary to explicitly request
-MPI builds. A user can request to use a particular MPI library by using the MPI_PATH variable. MPI support can be explicitely disabled by adding the -DNO_MPI=1
-flag to the cmake command line.
+Using the cmake method also has the advantage that the build is automatically checking for MPI installations. The tests can be compiled with MPI support by adding the `-DUSE_MPI=ON` flag to the cmake command line. A user can request to use a particular MPI library by setting the environment variable `MPI_HOME` or add the path of the MPI library to the cmake prefix path with `-DCMAKE_PREFIX_PATH`.
 
 
 ## Usage