blockchain_stats: don't use gmtime_r on Windows

In some cases, it doesn't like it (I don't know the details).

Factor into a new epee function

contrib/epee/include/misc_os_dependent.h

@@ -122,6 +122,15 @@ namespace misc_utils
 		return boost::lexical_cast<std::string>(GetCurrentThreadId());
 #elif defined(__GNUC__)  
 		return boost::lexical_cast<std::string>(pthread_self());
+#endif
+	}
+
+	inline bool get_gmt_time(time_t t, struct tm &tm)
+	{
+#ifdef _WIN32
+		return gmtime_s(&tm, &t);
+#else
+		return gmtime_r(&t, &tm);
 #endif
 	}
 }

contrib/epee/src/mlog.cpp

@@ -40,6 +40,7 @@
 #include <boost/filesystem.hpp>
 #include <boost/algorithm/string.hpp>
 #include "string_tools.h"
+#include "misc_os_dependent.h"
 #include "misc_log_ex.h"
 
 #undef MONERO_DEFAULT_LOG_CATEGORY
@@ -58,12 +59,7 @@ static std::string generate_log_filename(const char *base)
   char tmp[200];
   struct tm tm;
   time_t now = time(NULL);
-  if
-#ifdef WIN32
-  (!gmtime_s(&tm, &now))
-#else
-  (!gmtime_r(&now, &tm))
-#endif
+  if (!epee::misc_utils::get_gmt_time(now, tm))
     snprintf(tmp, sizeof(tmp), "part-%u", ++fallback_counter);
   else
     strftime(tmp, sizeof(tmp), "%Y-%m-%d-%H-%M-%S", &tm);

src/blockchain_utilities/blockchain_stats.cpp

@@ -234,7 +234,7 @@ plot 'stats.csv' index "DATA" using (timecolumn(1,"%Y-%m-%d")):4 with lines, ''
     }
     time_t tt = blk.timestamp;
     char timebuf[64];
-    gmtime_r(&tt, &currtm);
+    epee::misc_utils::get_gmt_time(tt, currtm);
     if (!prevtm.tm_year)
       prevtm = currtm;
     // catch change of day

src/simplewallet/simplewallet.cpp

@@ -6727,11 +6727,7 @@ static std::string get_human_readable_timestamp(uint64_t ts)
     return "<unknown>";
   time_t tt = ts;
   struct tm tm;
-#ifdef WIN32
-  gmtime_s(&tm, &tt);
-#else
-  gmtime_r(&tt, &tm);
-#endif
+  epee::misc_utils::get_gmt_time(tt, tm);
   uint64_t now = time(NULL);
   uint64_t diff = ts > now ? ts - now : now - ts;
   strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", &tm);