The readline lib on OpenBSD redefines __attribute__ to empty if __STRICT_ANSI__...
#include <thread>
#if defined (__OpenBSD__) || defined(__NetBSD__)
+// If this is not undeffed, __attribute__ wil be redefined by /usr/include/readline/rlstdc.h
+#undef __STRICT_ANSI__
#include <readline/readline.h>
#include <readline/history.h>
#else
#include <unistd.h>
#if defined (__OpenBSD__) || defined(__NetBSD__)
+// If this is not undeffed, __attribute__ wil be redefined by /usr/include/readline/rlstdc.h
+#undef __STRICT_ANSI__
#include <readline/readline.h>
#else
#include <editline/readline.h>