Add the fmt library (#4065)

* Replaces AppendVPrintf with fmt::sprintf
* fmt::ArgList now used as a type safe alternative to varargs.
* Removed SIZE_T_FMT compatibility macros. fmt::sprintf is fully portable and supports %zu.
* Adds FLOG functions to log with fmt's native formatting style.

src/HTTP/HTTPServerConnection.cpp

@@ -74,7 +74,7 @@ void cHTTPServerConnection::Send(const void * a_Data, size_t a_Size)
 {
 	ASSERT(m_CurrentRequest != nullptr);
 	// We're sending in Chunked transfer encoding
-	SendData(Printf(SIZE_T_FMT_HEX "\r\n", a_Size));
+	SendData(fmt::format("{0:x}\r\n", a_Size));
 	SendData(a_Data, a_Size);
 	SendData("\r\n");
 }