add timegm function (inverse of gmtime), nonstandard

include/time.h

@@ -114,6 +114,9 @@ struct tm *getdate (const char *);
 int stime(time_t *);
 #endif
 
+#if defined(_GNU_SOURCE)
+time_t timegm(struct tm *);
+#endif
 
 #ifdef __cplusplus
 }

src/time/timegm.c

+#define _GNU_SOURCE
+#include <time.h>
+
+#include "__time.h"
+
+time_t timegm(struct tm *tm)
+{
+	return __tm_to_time(tm);
+}