Add a "fork" of musl as //fusl.

fusl/include/err.h

Index: fusl/include/err.h
diff --git a/fusl/include/err.h b/fusl/include/err.h
new file mode 100644
index 0000000000000000000000000000000000000000..9f5cb6b9e9fad0ff255f2dd1b505cfcec4658d26
--- /dev/null
+++ b/fusl/include/err.h
@@ -0,0 +1,25 @@
+#ifndef _ERR_H
+#define _ERR_H
+
+#include <features.h>
+#include <stdarg.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void warn(const char *, ...);
+void vwarn(const char *, va_list);
+void warnx(const char *, ...);
+void vwarnx(const char *, va_list);
+
+_Noreturn void err(int, const char *, ...);
+_Noreturn void verr(int, const char *, va_list);
+_Noreturn void errx(int, const char *, ...);
+_Noreturn void verrx(int, const char *, va_list);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif