Newer curl versions (as used on macOS via Homebrew) add attributes like
__attribute__ ((format(printf, a, b)))
to their `curl_*printf*` functions, which fails if we redefine `printf`
as e.g. `builtin_printf` (pulled in via library.h). We could disable
these checks via CURL_NO_FMT_CHECKS, but reordering the headers should
do the trick as well.
If libcurl is built with MultiSSL support (not the case for e.g.
Debian/Ubuntu, which ship separate, conflicting libraries), this allows
selecting the SSL/TLS backend libcurl uses.
Only older versions of OpenSSL and GnuTLS need special treatment, so we
now accept all other backends (e.g. "(SecureTransport) OpenSSL/1.1.1s"
on macOS).
Whenever we remove support for the affected versions of the mentioned
libraries, we can remove the corresponding *-threading plugin feature
and the code here.
To use SSL in curl, we need to initialize the SSL library in a thread-safe
manner and provide the appropriate callbacks. As we already do that in our
crypto plugins using these libraries, we depend on these features.
This implies that we need the same plugin enabled (openssl, gcrypt) as the
curl backend is configured to use to fetch from HTTPS URIs.
Tobias Brunner