Add option to test CMake itself against its hosting CMake

CMakeLists.txt

@@ -18,6 +18,10 @@ if(CMAKE_BOOTSTRAP)
   unset(CMAKE_BOOTSTRAP CACHE)
 endif()
 
+if(CMake_TEST_HOST_CMAKE)
+  get_filename_component(CMake_TEST_EXTERNAL_CMAKE "${CMAKE_COMMAND}" DIRECTORY)
+endif()
+
 if(NOT CMake_TEST_EXTERNAL_CMAKE)
   if(CMAKE_SYSTEM_NAME STREQUAL "HP-UX")
     message(FATAL_ERROR