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.

internal.go

@@ -0,0 +1,42 @@
+// +build !external
+
+package main
+
+import (
+	"embed"
+	"io/fs"
+	"log"
+)
+
+const binaryType = "internal"
+
+//go:embed data data/html data/web data/web/css data/web/js
+var loFS embed.FS
+
+//go:embed lang/common lang/admin lang/email lang/form lang/setup lang/pwreset
+var laFS embed.FS
+
+var langFS rewriteFS
+var localFS rewriteFS
+
+type rewriteFS struct {
+	fs     embed.FS
+	prefix string
+}
+
+func (l rewriteFS) Open(name string) (fs.File, error)          { return l.fs.Open(l.prefix + name) }
+func (l rewriteFS) ReadDir(name string) ([]fs.DirEntry, error) { return l.fs.ReadDir(l.prefix + name) }
+func (l rewriteFS) ReadFile(name string) ([]byte, error)       { return l.fs.ReadFile(l.prefix + name) }
+func FSJoin(elem ...string) string {
+	out := ""
+	for _, v := range elem {
+		out += v + "/"
+	}
+	return out[:len(out)-1]
+}
+
+func loadFilesystems() {
+	langFS = rewriteFS{laFS, "lang/"}
+	localFS = rewriteFS{loFS, "data/"}
+	log.Println("Using internal storage")
+}