If not icon file is specified use the default Python Applet icons.

Fixes 719303.

Lib/plat-mac/buildtools.py

@@ -289,6 +289,10 @@ def process_common_macho(template, progress, code, rsrcname, destname, is_update
                 icnsname = None
         else:
             plistname = None
+    if not icnsname:
+        dft_icnsname = os.path.join(sys.prefix, 'Resources/Python.app/Contents/Resources/PythonApplet.icns')
+        if os.path.exists(dft_icnsname):
+            icnsname = dft_icnsname
     if not os.path.exists(rsrcname):
         rsrcname = None
     if progress: