Abstracts away time()/stdlib.h into platform

Substitutes time() into a configurable platform interface to allow it to be
easily substituted.

library/net.c

@@ -32,6 +32,13 @@
 #error "This module only works on Unix and Windows, see MBEDTLS_NET_C in config.h"
 #endif
 
+#if defined(MBEDTLS_PLATFORM_C)
+#include "mbedtls/platform.h"
+#else
+#include <stdlib.h>
+#define mbedtls_time_t    time_t
+#endif
+
 #include "mbedtls/net.h"
 
 #include <string.h>
@@ -86,7 +93,6 @@ static int wsa_init_done = 0;
 #define MSVC_INT_CAST
 #endif
 
-#include <stdlib.h>
 #include <stdio.h>
 
 #include <time.h>