Log Message:
devel/pango: Make this cross-compile.

1. Make introspection an option, default on for native builds (so no
   change to native buildS), default off for cross builds.

2. Make the pango-view.1 plist entry conditional on native builds.
   Apparently it works by running `pango-view --help' at build-time
   and munging the output into a man page, which is not gonna fly in
   cross builds.