https://github.com/djcb/mu/pull/2224 (rebased)
https://bugs.gentoo.org/839918
--- a/lib/utils/mu-utils.cc
+++ b/lib/utils/mu-utils.cc
@@ -32,6 +32,7 @@
 #include <string.h>
 #include <iostream>
 #include <algorithm>
+#include <functional>
 #include <numeric>

 #include <glib.h>
