Lots of Linux distros have different names (libcurl-gnutls.so vs etc), and
version the symbols (curl_global_init@@CURL_LIBSSL_3), so it's more compatible
to just dlsym the basic entry points we need and just demand that libcurl is
installed at all.
Alternately: we'll use our own libcurl build, but we'll probably have to dump
SSL support to make this sane to do.