pv-wrap: Generalize SHARED_LIBRARY_GUARD_CONFIG handling
We didn't handle the case where SHARED_LIBRARY_GUARD_CONFIG is already set to a custom value. It could conceivably also be in /usr or /lib*, which need the same remapping.
Broken out from !337 (merged) because this is really an orthogonal bug fix. Reviewed-by: @denittis on !337 (merged).