Update libexpat

[AiraYumi]

Description

Can you bump libexpat? I don't understand why we keep using an 8 year old library.

[RunitaiLinden]

Thanks for the heads up.

Updating libraries is one of those common tasks that takes a back seat to bug fixes and feature improvements that have a demonstrable benefit, but keeping libraries up to date is definitely something we're trying to be better at, and there is in fact an internal initiative to take care of some of the worst offenders like libexpat.

[bennettgoble]

I've got new version of expat prepared and have rebuilt its dependent library APR:

• https://github.com/secondlife/3p-expat/pull/4
• https://github.com/secondlife/3p-apr_suite/pull/5

However, there is another dependency which I'd prefer to remove rather than continue supporting: xmlrpc-epi:

• https://github.com/secondlife/viewer/issues/1111

[bennettgoble]

Linux, windows build working. There's link issues on macOS which I am investigating.