Fix issue with $< automatic variable on FreeBSD

BSD make only evaluates $< for implicit rules, so building from the
repository won't work unless GNU make is installed and used, or we
replace affected uses like this.

src/stroke/Makefile.am

@@ -15,9 +15,9 @@ stroke_keywords.h: $(srcdir)/stroke_keywords.h.in
 		$(AM_V_GEN) \
 		sed \
 		-e "s:\@GPERF_LEN_TYPE\@:$(GPERF_LEN_TYPE):" \
-		$< > $@
+		$(srcdir)/stroke_keywords.h.in > $@
 
 stroke_keywords.c:	$(srcdir)/stroke_keywords.txt stroke_keywords.h
 		$(AM_V_GEN) \
 		$(GPERF) -m 10 -D -C -G -t \
-			--output-file=$@ $<
+			--output-file=$@ $(srcdir)/stroke_keywords.txt