If prefix=/opt/jhbuild or something, it's wrong to try installing the nautilus extension to /usr/lib

The correct approach is that nautilus will search for its extensions on a path, such as XDG_DATA_DIRS or LD_LIBRARY_PATH

If nautilus is in the same prefix as evince, then there's no point getting the special extensiondir from the nautilus .pc file.

If nautilus is in a different prefix, it's wrong to write to that prefix if it's not the prefix passed to configure. Instead, someone running nautilus with this setup has to set env variables so nautilus can find all extensions on a search path.