Fix #ifdef inconsistency

fixes #310

Actually all key exchanges that use a certificate use signatures too, and
there is no key exchange that uses signatures but no cert, so merge those two
flags.

Conflicts:
	ChangeLog
diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 32eae0f..7463a48 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1923,7 +1923,7 @@
           MBEDTLS_KEY_EXCHANGE_RSA_PSK_ENABLED */
 
 #if defined(MBEDTLS_SSL_PROTO_TLS1_2)
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
 static int ssl_parse_signature_algorithm( mbedtls_ssl_context *ssl,
                                           unsigned char **p,
                                           unsigned char *end,
@@ -1979,7 +1979,7 @@
 
     return( 0 );
 }
-#endif /* MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED */
+#endif /* MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED */
 #endif /* MBEDTLS_SSL_PROTO_TLS1_2 */
 
 #if defined(MBEDTLS_KEY_EXCHANGE_ECDH_RSA_ENABLED) || \