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/library/version_features.c b/library/version_features.c
index 5cbe8ac..50afe1e 100644
--- a/library/version_features.c
+++ b/library/version_features.c
@@ -99,6 +99,9 @@
 #if defined(MBEDTLS_DES_ALT)
     "MBEDTLS_DES_ALT",
 #endif /* MBEDTLS_DES_ALT */
+#if defined(MBEDTLS_GCM_ALT)
+    "MBEDTLS_GCM_ALT",
+#endif /* MBEDTLS_GCM_ALT */
 #if defined(MBEDTLS_XTEA_ALT)
     "MBEDTLS_XTEA_ALT",
 #endif /* MBEDTLS_XTEA_ALT */