use build constraints for embed, clean up makefile

internal-files/external-files and compile-debug are gone, the
environment variables INTERNAL=on/off and DEBUG=on/off replace them.

external.go

@@ -0,0 +1,37 @@
+// +build external
+
+package main
+
+import (
+	"io/fs"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+const binaryType = "external"
+
+var localFS fs.FS
+var langFS fs.FS
+
+// When using os.DirFS, even on Windows the separator seems to be '/'.
+// func FSJoin(elem ...string) string { return filepath.Join(elem...) }
+func FSJoin(elem ...string) string {
+	sep := "/"
+	if strings.Contains(elem[0], "\\") {
+		sep = "\\"
+	}
+	path := ""
+	for _, el := range elem {
+		path += el + sep
+	}
+	return strings.TrimSuffix(path, sep)
+}
+
+func loadFilesystems() {
+	log.Println("Using external storage")
+	executable, _ := os.Executable()
+	localFS = os.DirFS(filepath.Join(filepath.Dir(executable), "data"))
+	langFS = os.DirFS(filepath.Join(filepath.Dir(executable), "data", "lang"))
+}