Misleading examples with `redacted`

Hi!

I see a couple of examples like examples/third_party/python/BUILD.python3.bazel that have this:

```
configure_options = [
"CFLAGS='-Dredacted=\"redacted\"'",
"--with-openssl=$EXT_BUILD_DEPS/openssl",
"--with-zlib=$EXT_BUILD_DEPS/zlib",
"--enable-optimizations",
```

Adding `CFLAGS` like that is misleading and can cause a lot of headaches, because then the toolchain-provided `CFLAGS` are overwritten. The `configure` command becomes:

`CFLAGS="" ./configure CFLAGS='-Dredacted=\"redacted\"'`

Would it be possible to change the example such that the behavior is preserved while not removing the toolchain-provided flags?

Start with examples/third_party/python/BUILD.python3.bazel and inspect how its configure_options are used to form the configure command. Compare the resulting CFLAGS handling with the toolchain-provided flags; done means the example preserves those flags while retaining the stated behavior.

Written by the indexing model from the issue text.