Make Makefile more configurable (doc and zsh dirs)

Some distributions expect the documentation in /usr/share/doc/name, and
others in /usr/share/doc/name-version. The zsh directory for completion
is also configurable due to differences between distributions.

Makefile

 PREFIX=/usr
+DOCDIR=$(PREFIX)/share/doc/$(self)
+ZSHDIR=$(PREFIX)/share/zsh/vendor-completions
 
 self=vcsh
 manpages=$(self).1
@@ -11,11 +13,11 @@ install: all
 	install -m 0755 $(self) $(DESTDIR)$(PREFIX)/bin
 	install -d $(DESTDIR)$(PREFIX)/share/man/man1
 	install -m 0644 $(manpages) $(DESTDIR)$(PREFIX)/share/man/man1
-	install -d $(DESTDIR)$(PREFIX)/share/doc/$(self)
-	install -m 0644 README.md $(DESTDIR)$(PREFIX)/share/doc/$(self)
-	install -m 0644 doc/hooks $(DESTDIR)$(PREFIX)/share/doc/$(self)
-	install -d $(DESTDIR)$(PREFIX)/share/zsh/vendor-completions
-	install -m 0644 _$(self) $(DESTDIR)$(PREFIX)/share/zsh/vendor-completions
+	install -d $(DESTDIR)$(DOCDIR)
+	install -m 0644 README.md $(DESTDIR)$(DOCDIR)
+	install -m 0644 doc/hooks $(DESTDIR)$(DOCDIR)
+	install -d $(DESTDIR)$(ZSHDIR)
+	install -m 0644 _$(self) $(DESTDIR)$(ZSHDIR)
 
 manpages: $(manpages)
 
@@ -28,16 +30,16 @@ clean:
 uninstall:
 	rm -rf $(DESTDIR)$(PREFIX)/bin/$(self)
 	rm -rf $(DESTDIR)$(PREFIX)/share/man/man1/$(self).1
-	rm -rf $(DESTDIR)$(PREFIX)/share/doc/$(self)
-	rm -rf $(DESTDIR)$(PREFIX)/share/zsh/vendor-completions/_$(self)
+	rm -rf $(DESTDIR)$(DOCDIR)
+	rm -rf $(DESTDIR)$(ZSHDIR)/_$(self)
 
 # Potentially harmful, used a non-standard option on purpose.
 # If PREFIX=/usr/local and that's empty...
 purge: uninstall
 	rmdir -p --ignore-fail-on-non-empty $(DESTDIR)$(PREFIX)/bin/
 	rmdir -p --ignore-fail-on-non-empty $(DESTDIR)$(PREFIX)/share/man/man1/
-	rmdir -p --ignore-fail-on-non-empty $(DESTDIR)$(PREFIX)/share/doc/
-	rmdir -p --ignore-fail-on-non-empty $(DESTDIR)$(PREFIX)/share/zsh/vendor-completions/
+	rmdir -p --ignore-fail-on-non-empty $(DESTDIR)$(shell dirname $(DOCDIR))
+	rmdir -p --ignore-fail-on-non-empty $(DESTDIR)$(ZSHDIR)
 
 test:
 	if which git > /dev/null ; then :; else echo "'git' not found, exiting..."; exit 1; fi