Allow alternate implementation of GCM

Provide the ability to use an alternative implementation of GCM in place
of the library-provided implementation.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 47c7196..94bf0d1 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -267,6 +267,7 @@
 //#define MBEDTLS_BLOWFISH_ALT
 //#define MBEDTLS_CAMELLIA_ALT
 //#define MBEDTLS_DES_ALT
+//#define MBEDTLS_GCM_ALT
 //#define MBEDTLS_XTEA_ALT
 //#define MBEDTLS_MD2_ALT
 //#define MBEDTLS_MD4_ALT