Revive -flipBlack option

The -flipBlack option in the cairo version is now implemented the same
way as it was in XBoard, swapping white and black pieces in filpView mode.
This requires a set black piece images that are the upside-down versions
of the white ones.
Add Xiangqi piece images to project

Let the images install in themes/xiangqi.
Fix Makefile for install of Xiangqi pieces
Connect Ctrl key in WinBoard

The Control key is now checked during move entry, for excluding moves
during analysis, or for copying pieces when editing a positions. But the
WB front-end did not read it out yet!
Better fix of feature timeout

The previous fix of the 2nd engine's done=1 aborting the 1st engine's
feature timeout by starting the game caused problems with v1 engines,
or other engines that would never send done=1. It is now fixed by
explicitly testing for a pending feature timeout after resurrecting
the 1st engine, rather than having TwoMachinesEventIfReady wait for
initDone to be set by reception of done=1.
 Also refrain from freezing the UI between match games, as in TwoMachines
mode the UI is mostly disabled anyway.
Unreserve tourney game on exit during engine load

While waiting for the feature timeout we are not yet in TwoMachines mode,
so quitting XBoard at that stage would not unreserve the upcoming game.
A global flag 'startingEngine' similar to 'waitingForGame' now signals
this condition.
Only perform e.p. capture if there are rights

The heuristic for e.p. capture was such that any non-forward move with
a Pawn to an empty square would delete the piece behind its new location
(Xiangqi exempted). This is now limited to cases where EP_STATUS indicates
that the deleted piece was a Pawn that performed a double-push on the
previous move. (Or if there is unknown EP_STATUS.)
 This makes XBoard more generally suitable for use with variants that have
non-conventional Pawn moves (with legality testing off). To guarantee this
will not have any unexpected effects in normal Chess, this patch will only
be active if legality testing is off.
add test for pkg-config

exit configure in case we can't find it.