Add cs_timegm

Which is a slightly modified version from
https://stackoverflow.com/questions/283166/easy-way-to-convert-a-struct-tm-expressed-in-utc-to-time-t-type

PUBLISHED_FROM=b73f920ca42c45473c23337782e815306bdf69f1

mongoose.c

@@ -780,6 +780,41 @@ double cs_time(void) {
 #endif /* _WIN32 */
   return now;
 }
+
+double cs_timegm(const struct tm *tm) {
+  /* Month-to-day offset for non-leap-years. */
+  static const int month_day[12] = {0,   31,  59,  90,  120, 151,
+                                    181, 212, 243, 273, 304, 334};
+
+  /* Most of the calculation is easy; leap years are the main difficulty. */
+  int month = tm->tm_mon % 12;
+  int year = tm->tm_year + tm->tm_mon / 12;
+  int year_for_leap;
+  int64_t rt;
+
+  if (month < 0) { /* Negative values % 12 are still negative. */
+    month += 12;
+    --year;
+  }
+
+  /* This is the number of Februaries since 1900. */
+  year_for_leap = (month > 1) ? year + 1 : year;
+
+  rt =
+      tm->tm_sec /* Seconds */
+      +
+      60 *
+          (tm->tm_min /* Minute = 60 seconds */
+           +
+           60 * (tm->tm_hour /* Hour = 60 minutes */
+                 +
+                 24 * (month_day[month] + tm->tm_mday - 1 /* Day = 24 hours */
+                       + 365 * (year - 70)                /* Year = 365 days */
+                       + (year_for_leap - 69) / 4 /* Every 4 years is leap... */
+                       - (year_for_leap - 1) / 100 /* Except centuries... */
+                       + (year_for_leap + 299) / 400))); /* Except 400s. */
+  return rt < 0 ? -1 : (double) rt;
+}
 #ifdef MG_MODULE_LINES
 #line 1 "common/cs_endian.h"
 #endif

mongoose.h

@@ -1797,6 +1797,8 @@ void cs_hmac_sha1(const unsigned char *key, size_t key_len,
 #ifndef CS_COMMON_CS_TIME_H_
 #define CS_COMMON_CS_TIME_H_
 
+#include <time.h>
+
 /* Amalgamated: #include "common/platform.h" */
 
 #ifdef __cplusplus
@@ -1806,6 +1808,12 @@ extern "C" {
 /* Sub-second granularity time(). */
 double cs_time(void);
 
+/*
+ * Similar to (non-standard) timegm, converts broken-down time into the number
+ * of seconds since Unix Epoch.
+ */
+double cs_timegm(const struct tm *tm);
+
 #ifdef __cplusplus
 }
 #endif /* __cplusplus */