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

fusl/include/ucontext.h

Index: fusl/include/ucontext.h
diff --git a/fusl/include/ucontext.h b/fusl/include/ucontext.h
new file mode 100644
index 0000000000000000000000000000000000000000..3bb776ed6a86969a530a558ff81506cfc7ccbb4b
--- /dev/null
+++ b/fusl/include/ucontext.h
@@ -0,0 +1,25 @@
+#ifndef _UCONTEXT_H
+#define _UCONTEXT_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#include <features.h>
+
+#include <signal.h>
+
+#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
+#define NGREG (sizeof(gregset_t)/sizeof(greg_t))
+#endif
+
+struct __ucontext;
+
+int  getcontext(struct __ucontext *);
+void makecontext(struct __ucontext *, void (*)(void), int, ...);
+int  setcontext(const struct __ucontext *);
+int  swapcontext(struct __ucontext *, const struct __ucontext *);
+
+#ifdef __cplusplus
+}
+#endif
+#endif