tweak install.sh for jhbuild

Make two fixes to install.sh.

1) Do a proper DESTDIR install instead of bypassing DESTDIR and creating
   some empty directories to cause jhbuild to think that we did the
   right thing.  This means that 'jhbuild uninstall' will work properly.

2) Install the xml catalogs in $prefix/share/etc/xml because this is
   where gtk-doc will look for them.