build_helper() breaks on gnutls downloading

The build breaks when gnutls is not available on mirrors.dotsrc.org.
Since commit 34801af9, we exit early on errors, and thus the script won't try to download from ftp.gnutls.org:

build_helper excerpt:

    wget http://mirrors.dotsrc.org/gcrypt/gnutls/v3.1/gnutls-3.1.23.tar.xz
    if [ $? -ne 0 ]; then
      wget ftp://ftp.gnutls.org/gcrypt/gnutls/v3.1/gnutls-3.1.23.tar.xz
    fi