ENH: make sure source flags don't have to be set

Modules/UseSWIG.cmake

@@ -65,6 +65,9 @@ MACRO(SWIG_ADD_SOURCE_TO_MODULE name outfiles infile)
   GET_SOURCE_FILE_PROPERTY(swig_source_file_generated ${infile} GENERATED)
   GET_SOURCE_FILE_PROPERTY(swig_source_file_cplusplus ${infile} CPLUSPLUS)
   GET_SOURCE_FILE_PROPERTY(swig_source_file_flags ${infile} SWIG_FLAGS)
+  IF(${swig_source_file_flags} STREQUAL "NOTFOUND")
+    SET(swig_source_file_flags "")
+  ENDIF(${swig_source_file_flags} STREQUAL "NOTFOUND")
   SET(swig_source_file_fullname "${infile}")
   IF(${swig_source_file_path} MATCHES "^${CMAKE_CURRENT_SOURCE_DIR}")
     STRING(REGEX REPLACE