Blender Git Loki

Deps: Add PugiXML as an official dependency

PugiXML was historically shipped hidden embedded into OIIO, the Grease
Pencil 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.

This commit expands the Windows-specific code in rBdca9aa0053f7 to
include Linux. macOS support will be handled in a later commit.

NOTE: run `cmake -U'*PUGIXML*' .` in the build directory to ensure CMake
finds PugiXML in the new location.

For details see D8628