aria2 with Apple's TLS by default

aria2 requires C++11, so its C++ dependencies (gnutls, gmp) must have C++11 options as well, unless we use Apple's built-in TLS support instead.

Based on configure output, there doesn't appear to be any difference in what functionality is enabled when using Apple's TLS vs. GnuTLS.

As a bonus, curl-ca-bundle isn't necessary, since it will have access to certificates in the system keychain.

If there is really a desire for a gnutls option, then someone will have to enable C++11 support in gnutls, and gmp (and potentially others).