Fix #10 — `RECALL` macro conflicts with mainline.

In Battle for Wesnoth commit 29bc800600f38367b96650a8152e4528ac68a62d,
Groggy Dice added a `RECALL` macro to mainline that conflicted with NX’s
`RECALL` macro, by taking a unit id only, whereas NX’s `RECALL` macro
also took coordinates at which to recall the unit.

NX was nice enough to not try to force its opinion of what a `RECALL`
macro should be by redefining it, so this conflict resulted in
preprocessor errors in NX due to the mainline `RECALL` macro having a
different parameter signature than NX’s `RECALL` macro, taking one
argument rather than three.

This commit resolves the issue by renaming NX’s `RECALL` macro to
`RECALL_XY`, as proposed by vultraz at 2014-01-02 20:52 UTC.