Merge pull request #3710 from blattms/enforce-64bit

Make CMake fail on non-64bit systems.

cmake/Modules/OpmInit.cmake

@@ -110,6 +110,10 @@ macro (OpmInitDirVars)
   endif (COMMAND dir_hook)
 endmacro ()
 
+if("${CMAKE_SIZEOF_VOID_P}" LESS 8)
+  message(FATAL_ERROR "OPM will only work correctly on 64bit (or higher) systems!")
+endif()
+
 OpmInitProjVars ()
 OpmInitDirVars ()