Use zlib submodule with "wx_zlib_" prefix

This will allow projects to use static wxWidgets libraries and zlib
(either directly or indirectly, as can heppen e.g. via Boost.IOStreams)
at the same time without link conflicts.

See #15314.

docs/changes.txt

@@ -104,6 +104,7 @@ All:
 - Don't crash in wxFFile::Eof() or Error() on closed file (jprotopopov).
 - Add UTF-8 support to wxZipOutputStream (Tobias Taschner).
 - Update all bundled 3rd party libraries to their latest versions.
+- Use unique prefix for all zlib symbols to avoid link conflicts.
 
 All (GUI):
 

src/zlib

@@ -1 +1 @@
-Subproject commit 1c1550729c409333f3cf4705a995c460599547a1
+Subproject commit 5888671274cde770edbe683b435f052de2b03681