Log Message:
x11/gtk3: Make this cross-compile.

New option `doc' for documentation.  Default-on for native builds
(hence no change to native builds), default-off for cross-builds
because apparently building the documentation requires running the
new gtk3 code and I can't figure out how to circumvent that in a few
minutes of fiddling.

New option `introspection' for gobject-introspection support.
Default-on for native builds (hence no change to native builds),
default-off for cross-builds (to which gobject-introspection is
fundamentally hostile).