Makefile.am: fix install-data-hook

- Prefix $(appdefaultdir) and $(datarootdir) with $(DESTDIR) to allow
  cross-compilation
- Create $(DESTDIR)$(appdefaultdir) if it doesn't exist

Upstream status: sent to Tom Sato <VEF00200@nifty.com>

diff -Nuar xvkbd-4.0-orig/Makefile.am xvkbd-4.0/Makefile.am
--- xvkbd-4.0-orig/Makefile.am	2019-10-06 11:27:13.692243604 +0200
+++ xvkbd-4.0/Makefile.am	2019-10-06 11:35:15.872263440 +0200
@@ -17,11 +17,12 @@
 	[ which ad2c > /dev/null ] || ad2c XVkbd-common.ad > XVkbd-common.h
 
 install-data-hook:
+	[ -d $(DESTDIR)$(appdefaultdir) ] || mkdir $(DESTDIR)$(appdefaultdir)
 	for file in *.ad; do \
-		$(INSTALL_DATA) $$file $(appdefaultdir)/$${file%.ad}; \
+		$(INSTALL_DATA) $$file $(DESTDIR)$(appdefaultdir)/$${file%.ad}; \
 	done
-	[ -d $(datarootdir)/xvkbd/ ] || mkdir $(datarootdir)/xvkbd/
-	$(INSTALL_DATA) words.english $(datarootdir)/xvkbd/words.english
+	[ -d $(DESTDIR)$(datarootdir)/xvkbd/ ] || mkdir $(DESTDIR)$(datarootdir)/xvkbd/
+	$(INSTALL_DATA) words.english $(DESTDIR)$(datarootdir)/xvkbd/words.english
 
 EXTRA_DIST = XVkbd-common.h *.ad *.xbm xvkbd.man README ChangeLog Imakefile words.english