Fix std.outbuffer.[v]printf() for Visual Studio 2015+

MS conforms to the standard beginning with VS 2015, i.e., if the buffer is too small,
vsnprintf() returns the number of required characters (excl. terminating null). VS 2013
and earlier always returned -1 in that case.
So just use the generic (previous POSIX) version, it's compatible with older VS anyway.

Already unittested in line 380 ("hello world 6" vs. "hello world 62665").

Author: kinke

std/outbuffer.d

@@ -256,45 +256,24 @@ class OutBuffer
         auto psize = buffer.length;
         for (;;)
         {
-            version(Windows)
+            va_list args2;
+            va_copy(args2, args);
+            count = vsnprintf(p, psize, f, args2);
+            va_end(args2);
+            if (count == -1)
             {
-                va_list args2;
-                va_copy(args2, args);
-                count = vsnprintf(p,psize,f,args2);
-                va_end(args2);
-                if (count != -1)
-                    break;
-
                 if (psize > psize.max / 2) assert(0); // overflow check
                 psize *= 2;
-
-                p = cast(char *) alloca(psize); // buffer too small, try again with larger size
             }
-            else version(Posix)
+            else if (count >= psize)
             {
-                va_list args2;
-                va_copy(args2, args);
-                count = vsnprintf(p, psize, f, args2);
-                va_end(args2);
-                if (count == -1)
-                {
-                    if (psize > psize.max / 2) assert(0); // overflow check
-                    psize *= 2;
-                }
-                else if (count >= psize)
-                {
-                    if (count == count.max) assert(0); // overflow check
-                    psize = count + 1;
-                }
-                else
-                    break;
-
-                p = cast(char *) alloca(psize); // buffer too small, try again with larger size
+                if (count == count.max) assert(0); // overflow check
+                psize = count + 1;
             }
             else
-            {
-                static assert(0);
-            }
+                break;
+
+            p = cast(char *) alloca(psize); // buffer too small, try again with larger size
         }
         write(cast(ubyte[]) p[0 .. count]);
     }