--- butt-1.44.0.orig/src/FLTK/FL/Fl_My_Native_File_Chooser.H
+++ butt-1.44.0.orig/src/FLTK/FL/Fl_My_Native_File_Chooser.H
@@ -40,6 +40,7 @@
 #if !defined(__APPLE__) && !defined(WIN32)
+#include <cstdlib>
 #include <FL/Fl_File_Chooser.H>
 #else
 #include <FL/filename.H> // FL_EXPORT