PugiXML was historically shipped hidden embedded into OIIO, the GP team had a requirement for an XML library recently so pugi seems like a natural choice since it's not really a 'new' library, we just turn an implicit dependency into an explicit one.
1) Updates the builder to build pugiXML on all platforms (previously just windows)
2) Disabled the embedded copy inside OIIO
3) Adds some checks to disable components when PUGI is not available.
I tested windows and linux but given linux is not my primary platform a second look would be appreciated there.
I was unable to test on mac (or arm mac)
This diff is a prerequisite for work the GP team would like to land in 2.91