zlib spkg fails to build on macOS 15.5

[Marc Culler]

The sage zlib package fails to build on macOS 15.5.  The reason is that version 1.2.11.p0 that sage uses assumes that fdopen is a macro, which evidently is no longer true with the MacOSX15.4 SDK.  The zlib code then replaces the non-existent fdopen macro with its own version, which does not work.  A simple fix is to upgrade the zlib package to the current version 1.3.1 which builds with no issues.

I realize that there is work in progress to remove the zlib spkg.  But updating the current one is much simpler - there is no need to construct a phony zlib.pc file and make sure sage can find it.  So as a temporary stopgap while the other project works its way through the system,  I suggest updating the zlib version to 1.3.1.

- Marc