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

fusl/src/time/timegm.c

Index: fusl/src/time/timegm.c
diff --git a/fusl/src/time/timegm.c b/fusl/src/time/timegm.c
new file mode 100644
index 0000000000000000000000000000000000000000..b5dae8b648eca781ca9e726f90d5b6cc85b72d1a
--- /dev/null
+++ b/fusl/src/time/timegm.c
@@ -0,0 +1,20 @@
+#define _GNU_SOURCE
+#include "time_impl.h"
+#include <errno.h>
+
+extern const char __gmt[];
+
+time_t timegm(struct tm *tm)
+{
+	struct tm new;
+	long long t = __tm_to_secs(tm);
+	if (__secs_to_tm(t, &new) < 0) {
+		errno = EOVERFLOW;
+		return -1;
+	}
+	*tm = new;
+	tm->tm_isdst = 0;
+	tm->__tm_gmtoff = 0;
+	tm->__tm_zone = __gmt;
+	return t;
+}