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

fusl/src/stdio/vasprintf.c

Index: fusl/src/stdio/vasprintf.c
diff --git a/fusl/src/stdio/vasprintf.c b/fusl/src/stdio/vasprintf.c
new file mode 100644
index 0000000000000000000000000000000000000000..08251bc20ecd240a53335357341f89000ce5efa6
--- /dev/null
+++ b/fusl/src/stdio/vasprintf.c
@@ -0,0 +1,15 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdarg.h>
+#include <stdlib.h>
+
+int vasprintf(char **s, const char *fmt, va_list ap)
+{
+	va_list ap2;
+	va_copy(ap2, ap);
+	int l = vsnprintf(0, 0, fmt, ap2);
+	va_end(ap2);
+
+	if (l<0 || !(*s=malloc(l+1U))) return -1;
+	return vsnprintf(*s, l+1U, fmt, ap);
+}