fs: Treat . and .. in userdata/config paths relative to cwd on Windows

This allows asking Wesnoth to store user files in a directory relative
to the install dir if wanted on Windows (e.g. for portable installs to
removable media where the absolute path to the install dir is not
constant or under the user's control).

src/filesystem_boost.cpp

@@ -478,12 +478,32 @@ static void setup_user_data_dir()
 	create_directory_if_missing(user_data_dir / "saves");
 	create_directory_if_missing(user_data_dir / "persist");
 }
+
+#ifdef _WIN32
+// As a convenience for portable installs on Windows, relative paths with . or
+// .. as the first component are considered relative to the current workdir
+// instead of Documents/My Games.
+static bool is_path_relative_to_cwd(const std::string& str)
+{
+	const path p(str);
+
+	if(p.empty()) {
+		return false;
+	}
+
+	return *p.begin() == "." || *p.begin() == "..";
+}
+#endif
+
 void set_user_data_dir(std::string newprefdir)
 {
 #ifdef _WIN32
 	if(newprefdir.size() > 2 && newprefdir[1] == ':') {
 		//allow absolute path override
 		user_data_dir = newprefdir;
+	} else if(is_path_relative_to_cwd(newprefdir)) {
+		// Custom directory relative to workdir (for portable installs, etc.)
+		user_data_dir = get_cwd() + "/" + newprefdir;
 	} else {
 		if(newprefdir.empty()) {
 			newprefdir = "Wesnoth" + get_version_path_suffix();