Fix DLL search path on Windows with Python 3.8

Python 3.8 no longer looks for DLLs in the insecure
search path, i.e. it ignores %PATH%. Add the necessary
directory using os.add_dll_directory() to make this
work again.

tools/w32/add_search_path.py

@@ -39,6 +39,9 @@ def _add_pyext_to_path():
         # Add DLL directory to PATH so Windows can find them
         if dlldir not in os.environ['PATH']:
             os.environ['PATH'] = dlldir + ';' + os.environ['PATH']
+        # Python 3.8 or later don't look in PATH for DLLs
+        if hasattr(os, 'add_dll_directory'):
+            __dll_directory = os.add_dll_directory(dlldir)
 _add_pyext_to_path()
 
 """
@@ -60,6 +63,9 @@ def _add_pyext_to_path():
         # Add DLL directory to PATH so Windows can find them
         if dlldir not in os.environ['PATH']:
             os.environ['PATH'] = dlldir + ';' + os.environ['PATH']
+        # Python 3.8 or later don't look in PATH for DLLs
+        if hasattr(os, 'add_dll_directory'):
+            __dll_directory = os.add_dll_directory(dlldir)
 _add_pyext_to_path()
 
 """