Abstracts away time()/stdlib.h into platform

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

programs/ssl/mini_client.c

@@ -43,12 +43,14 @@
 #if !defined(MBEDTLS_CTR_DRBG_C) || !defined(MBEDTLS_ENTROPY_C) || \
     !defined(MBEDTLS_NET_C) || !defined(MBEDTLS_SSL_CLI_C) || \
     !defined(UNIX)
+
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
 #define mbedtls_printf printf
 #endif
+
 int main( void )
 {
     mbedtls_printf( "MBEDTLS_CTR_DRBG_C and/or MBEDTLS_ENTROPY_C and/or "
@@ -58,6 +60,15 @@ int main( void )
 }
 #else
 
+#if defined(MBEDTLS_PLATFORM_C)
+#include "mbedtls/platform.h"
+#else
+#include <stdlib.h>
+#define mbedtls_time_t       time_t
+#define MBEDTLS_EXIT_SUCCESS EXIT_SUCCESS
+#define MBEDTLS_EXIT_FAILURE EXIT_FAILURE
+#endif
+
 #include <string.h>
 
 #include "mbedtls/net.h"