UseSWIG.cmake did not support multiple modules and parallel builds

This commit fixes BUG: 0011782. UseSWIG would be using the same variable
to declare module information. The problem would only be noticed in parallel builds
Fix this variable declaration by properly resetting it.

Modules/UseSWIG.cmake

@@ -69,6 +69,7 @@ ENDMACRO(SWIG_MODULE_INITIALIZE)
 #
 
 MACRO(SWIG_GET_EXTRA_OUTPUT_FILES language outfiles generatedpath infile)
+  SET(${outfiles} "")
   GET_SOURCE_FILE_PROPERTY(SWIG_GET_EXTRA_OUTPUT_FILES_module_basename
     ${infile} SWIG_MODULE_NAME)
   IF(SWIG_GET_EXTRA_OUTPUT_FILES_module_basename STREQUAL "NOTFOUND")