Make windows_proof_rmtree resilient to read-only files

Start the process by traversing the tree and adding the S_IWRITE and
S_IREAD bits to the file's mode (which are also preserved on Windows.)

This fixes windows_proof_rmtree's inability to remove read-only files,
which was uncovered in testing the new `install_mode` feature.

Tested: ./run_tests.py passes on Linux, appveyor CI on Windows passes.

mesonbuild/mesonlib.py

@@ -975,12 +975,25 @@ def get_filenames_templates_dict(inputs, outputs):
     return values
 
 
+def _make_tree_writable(topdir):
+    # Ensure all files and directories under topdir are writable
+    # (and readable) by owner.
+    for d, _, files in os.walk(topdir):
+        os.chmod(d, os.stat(d).st_mode | stat.S_IWRITE | stat.S_IREAD)
+        for fname in files:
+            fpath = os.path.join(d, fname)
+            if os.path.isfile(fpath):
+                os.chmod(fpath, os.stat(fpath).st_mode | stat.S_IWRITE | stat.S_IREAD)
+
+
 def windows_proof_rmtree(f):
     # On Windows if anyone is holding a file open you can't
     # delete it. As an example an anti virus scanner might
     # be scanning files you are trying to delete. The only
     # way to fix this is to try again and again.
     delays = [0.1, 0.1, 0.2, 0.2, 0.2, 0.5, 0.5, 1, 1, 1, 1, 2]
+    # Start by making the tree wriable.
+    _make_tree_writable(f)
     for d in delays:
         try:
             shutil.rmtree(f)