meson: Add a new option to control the source for orc

Previously we were unconditionally cloning the orc subproject because we want the developer environment to use the latest orc. However, some people want to use the system orc instead, or want to auto-detect which to use, with the system orc getting preference. This requires adding a new option to select that. See discussion at: !2556 (closed)

Fixes #1282 (closed)