CommandLine: show the current git tag and commit id

configure.ac

@@ -15,6 +15,10 @@ AC_CONFIG_MACRO_DIR([m4])
 
 AC_DEFINE(PROTOCOL_VERSION, "0.19.0", [The MPD protocol version])
 
+GIT_COMMIT=`GIT_DIR="$srcdir/.git" git describe --dirty --always 2>/dev/null`
+if test x$GIT_COMMIT != x; then
+	AC_DEFINE_UNQUOTED(GIT_COMMIT, ["$GIT_COMMIT"], [The current git commit])
+fi
 
 dnl ---------------------------------------------------------------------------
 dnl Programs

src/CommandLine.cxx

@@ -88,7 +88,11 @@ static constexpr Domain cmdline_domain("cmdline");
 gcc_noreturn
 static void version(void)
 {
-	puts("Music Player Daemon " VERSION "\n"
+	puts("Music Player Daemon " VERSION
+#ifdef GIT_COMMIT
+	     " (" GIT_COMMIT ")"
+#endif
+	     "\n"
 	     "\n"
 	     "Copyright (C) 2003-2007 Warren Dukes <warren.dukes@gmail.com>\n"
 	     "Copyright (C) 2008-2014 Max Kellermann <max@duempel.org>\n"