Abstracts away time()/stdlib.h into platform

Substitutes time() into a configurable platform interface to allow it to be
easily substituted.
diff --git a/library/x509.c b/library/x509.c
index ffc3d6c..a0df817 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -53,10 +53,12 @@
 #else
 #include <stdio.h>
 #include <stdlib.h>
-#define mbedtls_free       free
+#define mbedtls_free      free
 #define mbedtls_calloc    calloc
-#define mbedtls_printf     printf
-#define mbedtls_snprintf   snprintf
+#define mbedtls_time      time
+#define mbedtls_time_t    time_t
+#define mbedtls_printf    printf
+#define mbedtls_snprintf  snprintf
 #endif
 
 #if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
@@ -843,7 +845,7 @@
 static int x509_get_current_time( mbedtls_x509_time *now )
 {
     struct tm *lt;
-    time_t tt;
+    mbedtls_time_t tt;
     int ret = 0;
 
 #if defined(MBEDTLS_THREADING_C)
@@ -851,7 +853,7 @@
         return( MBEDTLS_ERR_THREADING_MUTEX_ERROR );
 #endif
 
-    tt = time( NULL );
+    tt = mbedtls_time( NULL );
     lt = gmtime( &tt );
 
     if( lt == NULL )