Fix and simplify *-PSK ifdef's
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 6db92d6..79c63fd 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2353,9 +2353,7 @@
 }
 #endif /* POLARSSL_KEY_EXCHANGE_RSA_ENABLED */
 
-#if defined(POLARSSL_KEY_EXCHANGE_PSK_ENABLED) ||                           \
-    defined(POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED) ||                       \
-    defined(POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
+#if defined(POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED)
 static int ssl_parse_client_psk_identity( ssl_context *ssl, unsigned char **p,
                                           const unsigned char *end )
 {
@@ -2421,9 +2419,7 @@
 
     return( ret );
 }
-#endif /* POLARSSL_KEY_EXCHANGE_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED ||
-          POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED */
+#endif /* POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED */
 
 static int ssl_parse_client_key_exchange( ssl_context *ssl )
 {