FindSWIG: allow to provide SWIG_DIR (#3498)

Modules/FindSWIG.cmake

@@ -52,8 +52,6 @@ if(SWIG_EXECUTABLE)
     endif()
   else()
     string(REGEX REPLACE "[\n\r]+" ";" SWIG_swiglib_output ${SWIG_swiglib_output})
-    # force the path to be computed each time in case SWIG_EXECUTABLE has changed.
-    set(SWIG_DIR SWIG_DIR-NOTFOUND)
     find_path(SWIG_DIR swig.swg PATHS ${SWIG_swiglib_output} NO_CMAKE_FIND_ROOT_PATH)
     if(SWIG_DIR)
       set(SWIG_USE_FILE ${CMAKE_CURRENT_LIST_DIR}/UseSWIG.cmake)