Make CMake fail on non-64bit systems.

Running simulations on those does not make sense as it seriously
limits the time span that can be simulated. In addition the OPM code
neglects 32bit issues at various places as it was written at a time
where 64bit was already ubiquitous.

Currently, we would fail in some regression tests on 32bit
systems. With this change CMake will already check whether it is run
on a 64bit system and already fail if this is not the case.

cmake/Modules/OpmInit.cmake

@@ -110,6 +110,10 @@ macro (OpmInitDirVars)
   endif (COMMAND dir_hook)
 endmacro ()
 
+if("${CMAKE_SIZEOF_VOID_P}" STREQUAL "4")
+  message(FATAL_ERROR "OPM will only work correctly on 64bit systems!")
+endif()
+
 OpmInitProjVars ()
 OpmInitDirVars ()