Fix build on linux, enable build directory support

Specifically:

- Include stddef.h for missing ptrdiff_t
- Define PCAP_OPENFLAG_PROMISCUOUS if not using WinPcap because it seems to be
  a WinPcap extension
- Make use of CMAKE_CURRENT_{SOURCE,BINARY}_DIR to enable building under a
  directory other than root of the project

src/WifiAP.cpp

@@ -23,6 +23,9 @@
 #include "WifiAP.h"
 #include "Platform.h"
 
+#ifndef __WIN32__
+#include <stddef.h>
+#endif
 
 namespace WifiAP
 {

src/libui_sdl/Platform.cpp

@@ -38,6 +38,7 @@
 	#define socket_t    int
 	#define sockaddr_t  struct sockaddr
 	#define closesocket close
+	#define PCAP_OPENFLAG_PROMISCUOUS 1
 #endif
 
 #ifndef INVALID_SOCKET