Make function mbedtls_ssl_set_hostname(...) as optional

Now function mbedtls_ssl_set_hostname is compile-time configurable
in config.h with define MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION.
This affects to many x509 API's. See config.h for details.
diff --git a/library/x509.c b/library/x509.c
index a6c6584..19cc64b 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -1250,7 +1250,11 @@
     if( verbose != 0 )
         mbedtls_printf( "passed\n  X.509 signature verify: ");
 
-    ret = mbedtls_x509_crt_verify( &clicert, &cacert, NULL, NULL, &flags, NULL, NULL );
+    ret = mbedtls_x509_crt_verify( &clicert, &cacert, NULL,
+#if !defined(MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION)
+    NULL,
+#endif /* !MBEDTLS_X509_REMOVE_HOSTNAME_VERIFICATION */
+    &flags, NULL, NULL );
     if( ret != 0 )
     {
         if( verbose != 0 )