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.
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 9142be8..1e3b363 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -5800,7 +5800,7 @@
 }
 #endif /* MBEDTLS_DHM_C && MBEDTLS_SSL_CLI_C */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
 /*
  * Set allowed/preferred hashes for handshake signatures
  */
@@ -6981,7 +6981,7 @@
     0
 };
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
 static int ssl_preset_suiteb_hashes[] = {
     MBEDTLS_MD_SHA256,
     MBEDTLS_MD_SHA384,
@@ -7097,7 +7097,7 @@
             conf->cert_profile = &mbedtls_x509_crt_profile_suiteb;
 #endif
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
             conf->sig_hashes = ssl_preset_suiteb_hashes;
 #endif
 
@@ -7130,7 +7130,7 @@
             conf->cert_profile = &mbedtls_x509_crt_profile_default;
 #endif
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
             conf->sig_hashes = mbedtls_md_list();
 #endif
 
@@ -7294,7 +7294,7 @@
 }
 #endif /* MBEDTLS_ECP_C */
 
-#if defined(MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED)
+#if defined(MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED)
 /*
  * Check if a hash proposed by the peer is in our list.
  * Return 0 if we're willing to use it, -1 otherwise.
@@ -7313,7 +7313,7 @@
 
     return( -1 );
 }
-#endif /* MBEDTLS_KEY_EXCHANGE__SOME__SIGNATURE_ENABLED */
+#endif /* MBEDTLS_KEY_EXCHANGE__WITH_CERT__ENABLED */
 
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 int mbedtls_ssl_check_cert_usage( const mbedtls_x509_crt *cert,