Make RSA_ALT support optionnal
diff --git a/library/pk.c b/library/pk.c
index f083b86..d147302 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -112,6 +112,7 @@
     return( 0 );
 }
 
+#if defined(POLARSSL_PK_RSA_ALT_SUPPORT)
 /*
  * Initialize an RSA-alt context
  */
@@ -140,6 +141,7 @@
 
     return( 0 );
 }
+#endif /* POLARSSL_PK_RSA_ALT_SUPPORT */
 
 /*
  * Tell if a PK can do the operations of the given type