set CMake policy regarding leading/trailing whitespace

Appears with
   OpenMPI 1.10.4
   cmake 3.4.1

CMakeLists.txt

@@ -4,6 +4,10 @@ option(CM_ALL_VTK_OUTPUT "VTK output routine" OFF)
 option(CM_ALL_VORONOI "Voronoi-based loadbalancing scheme (requires Voro++)" OFF)
 option(CM_ALL_FORTRAN "VTK output routine" OFF)
 
+# leading and trailing whitespace should be silently ignored
+# thanks to an old FindMPI script
+cmake_policy(SET CMP0004 OLD)
+
 enable_language(CXX)
 
 if (CM_ALL_FORTRAN)