Move the benchmarking logic out into a script.

It's a pain having it in a rule in Makefile.gtk, which isn't even the
recommended makefile these days - it can't be re-run conveniently, and
there's no way to parametrise it. Now it can be run no matter which
makefile you're using, and it lets you narrow down to a subset of
games (though not presets). Other options could easily be added.

Recipe

@@ -142,11 +142,7 @@ test: benchmark.html benchmark.txt
 benchmark.html: benchmark.txt benchmark.pl
 	./benchmark.pl benchmark.txt > $@
 
-benchmark.txt: $(GAMES)
-	for i in $(GAMES); do \
-		for params in $$(env -i ./$(BINPREFIX)$$i --list-presets | cut -f1 -d' '); do \
-			env -i ./$(BINPREFIX)$$i --test-solve --time-generation --generate 100 $$params \
-			|| exit 1; \
-		done; \
-	done > $@
+benchmark.txt: benchmark.sh $(GAMES)
+	./benchmark.sh > $@
+
 !end