Adapt sources to configurable config.h name
diff --git a/library/pkcs12.c b/library/pkcs12.c
index 16821b0..637c057 100644
--- a/library/pkcs12.c
+++ b/library/pkcs12.c
@@ -29,7 +29,11 @@
  *  ftp://ftp.rsasecurity.com/pub/pkcs/pkcs-12/pkcs-12v1-1.asn
  */
 
+#if !defined(POLARSSL_CONFIG_FILE)
 #include "polarssl/config.h"
+#else
+#include POLARSSL_CONFIG_FILE
+#endif
 
 #if defined(POLARSSL_PKCS12_C)