configure.ac: Fix gnutls version check
This is version number is hexadecimal
See gnutls/gnutls.h
#define GNUTLS_VERSION "3.6.13"
#define GNUTLS_VERSION_MAJOR 3
#define GNUTLS_VERSION_MINOR 6
#define GNUTLS_VERSION_PATCH 13
#define GNUTLS_VERSION_NUMBER 0x03060d
Edited by Manuel Rüger