Revert the part of the change in r408688 that allowed the PDF Import
extension to build with boost 1.60, which should have been reverted
in r411540. The fix implemented upstream does not require linking
with any boost libraries, so return boost to always just being
a build dependency.

Update TARGET_ORDER_OVERRIDE to for the changes to Uses/gnome.mk
in r413768.

Don't bother trying to create ${STAGEDIR}${DESKTOPDIR} since it
is already handled by mtree.