FindSWIG: Do not generate erroneous outputs with -noproxy

Generally for a module foo, SWIG generates (in Python mode)
an extension module _foo.so and a proxy Python module foo.py.
However, if -noproxy is specified, instead it builds only foo.so
(without the leading underscore). The custom command generated
by CMake correctly handles the removal of this underscore when
-noproxy is given; however, it still adds foo.py to the expected
outputs. This upsets build tools that expect foo.py to be generated
(for example, 'make' will run the SWIG command twice). Fix this
by removing foo.py from the set of extra generated files when
-noproxy is specified.

Modules/UseSWIG.cmake

@@ -153,10 +153,12 @@ macro(SWIG_ADD_SOURCE_TO_MODULE name outfiles infile)
   else()
     set(swig_outdir ${CMAKE_CURRENT_BINARY_DIR})
   endif()
-  SWIG_GET_EXTRA_OUTPUT_FILES(${SWIG_MODULE_${name}_LANGUAGE}
-    swig_extra_generated_files
-    "${swig_outdir}"
-    "${swig_source_file_fullname}")
+  if (NOT SWIG_MODULE_${name}_NOPROXY)
+    SWIG_GET_EXTRA_OUTPUT_FILES(${SWIG_MODULE_${name}_LANGUAGE}
+      swig_extra_generated_files
+      "${swig_outdir}"
+      "${swig_source_file_fullname}")
+  endif()
   set(swig_generated_file_fullname
     "${swig_outdir}/${swig_source_file_name_we}")
   # add the language into the name of the file (i.e. TCL_wrap)