std.datetime fixed so that it doesn't need the daylight variable on P…

…osix.

It will still use daylight if it's there, but since FreeBSD doesn't have
it for some reason (in spite of the fact that the daylight variable is
part of Posix), we can't assume that it's there.

Also, now that tzname is in druntime, it has been removed from
std.datetime.

This should fix bug# 5616.

std/datetime.d

@@ -28472,18 +28472,6 @@ private:
 }
 
 
-version(Posix)
-{
-//This should be in core.stdc.time, but it isn't, so
-//we're declaring it here.
-extern(C)
-{
-    extern __gshared char* tzname[2];
-    extern __gshared int   daylight;
-}
-}
-
-
 /++
     A TimeZone which represents the current local time zone on
     the system running your program.
@@ -28685,7 +28673,25 @@ public:
     @property override bool hasDST() const nothrow
     {
         version(Posix)
-            return cast(bool)(daylight);
+        {
+            static if(is(typeof(daylight)))
+                return cast(bool)(daylight);
+            else
+            {
+                try
+                {
+                    auto currYear = (cast(Date)Clock.currTime()).year;
+                    auto janOffset = SysTime(Date(currYear, 1, 4), this).stdTime -
+                                     SysTime(Date(currYear, 1, 4), UTC()).stdTime;
+                    auto julyOffset = SysTime(Date(currYear, 7, 4), this).stdTime -
+                                      SysTime(Date(currYear, 7, 4), UTC()).stdTime;
+
+                    return janOffset != julyOffset;
+                }
+                catch(Exception e)
+                    assert(0, "Clock.currTime() threw.");
+            }
+        }
         else version(Windows)
         {
             try
@@ -28715,6 +28721,9 @@ public:
 
                 setTZEnvVar("America/New_York");
                 assert(LocalTime().hasDST);
+
+                setTZEnvVar("UTC");
+                assert(!LocalTime().hasDST);
             }
         }
     }