Adapt sources to configurable config.h name
diff --git a/include/polarssl/memory_buffer_alloc.h b/include/polarssl/memory_buffer_alloc.h
index 68477a4..c449752 100644
--- a/include/polarssl/memory_buffer_alloc.h
+++ b/include/polarssl/memory_buffer_alloc.h
@@ -27,7 +27,11 @@
 #ifndef POLARSSL_MEMORY_BUFFER_ALLOC_H
 #define POLARSSL_MEMORY_BUFFER_ALLOC_H
 
+#if !defined(POLARSSL_CONFIG_FILE)
 #include "config.h"
+#else
+#include POLARSSL_CONFIG_FILE
+#endif
 
 #include <stdlib.h>