Fix and simplify *-PSK ifdef's
diff --git a/include/polarssl/ssl.h b/include/polarssl/ssl.h
index b6944f1..94c996c 100644
--- a/include/polarssl/ssl.h
+++ b/include/polarssl/ssl.h
@@ -75,6 +75,14 @@
 #include <time.h>
 #endif
 
+/* For convenience below and in programs */
+#if defined(POLARSSL_KEY_EXCHANGE_PSK_ENABLED) ||                           \
+    defined(POLARSSL_KEY_EXCHANGE_RSA_PSK_ENABLED) ||                       \
+    defined(POLARSSL_KEY_EXCHANGE_DHE_PSK_ENABLED) ||                       \
+    defined(POLARSSL_KEY_EXCHANGE_ECDHE_PSK_ENABLED)
+#define POLARSSL_KEY_EXCHANGE__SOME__PSK_ENABLED
+#endif
+
 #if defined(_MSC_VER) && !defined(inline)
 #define inline _inline
 #else
@@ -614,9 +622,7 @@
     void *p_vrfy;               /*!< context for verification         */
 #endif
 
-#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)
     int (*f_psk)(void *, ssl_context *, const unsigned char *, size_t);
     void *p_psk;               /*!< context for PSK retrieval         */
 #endif
@@ -714,9 +720,7 @@
     mpi dhm_G;                          /*!<  generator for DHM       */
 #endif
 
-#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)
     /*
      * PSK values
      */
@@ -1058,9 +1062,7 @@
                           rsa_key_len_func rsa_key_len );
 #endif /* POLARSSL_X509_CRT_PARSE_C */
 
-#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)
 /**
  * \brief          Set the Pre Shared Key (PSK) and the identity name connected
  *                 to it.
@@ -1100,9 +1102,7 @@
                      int (*f_psk)(void *, ssl_context *, const unsigned char *,
                                   size_t),
                      void *p_psk );
-#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 */
 
 #if defined(POLARSSL_DHM_C)
 /**
@@ -1528,9 +1528,7 @@
 
 void ssl_optimize_checksum( ssl_context *ssl, const ssl_ciphersuite_t *ciphersuite_info );
 
-#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)
 int ssl_psk_derive_premaster( ssl_context *ssl, key_exchange_type_t key_ex );
 #endif