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

fusl/src/stdio/fgetln.c

Index: fusl/src/stdio/fgetln.c
diff --git a/fusl/src/stdio/fgetln.c b/fusl/src/stdio/fgetln.c
new file mode 100644
index 0000000000000000000000000000000000000000..afe12b5dfe024add1739f80d5982eefef76d7567
--- /dev/null
+++ b/fusl/src/stdio/fgetln.c
@@ -0,0 +1,21 @@
+#define _GNU_SOURCE
+#include "stdio_impl.h"
+#include <string.h>
+
+char *fgetln(FILE *f, size_t *plen)
+{
+	char *ret = 0, *z;
+	ssize_t l;
+	FLOCK(f);
+	ungetc(getc_unlocked(f), f);
+	if ((z=memchr(f->rpos, '\n', f->rend - f->rpos))) {
+		ret = (char *)f->rpos;
+		*plen = ++z - ret;
+		f->rpos = (void *)z;
+	} else if ((l = getline(&f->getln_buf, (size_t[]){0}, f)) > 0) {
+		*plen = l;
+		ret = f->getln_buf;
+	}
+	FUNLOCK(f);
+	return ret;
+}