Failing to do on clients was causing TLS 1.3 negotiation to fail.

Fixes bug 28245; bugfix on 0.2.9.15, when we added TLS 1.3 support.

Fixes bug 27344, where we'd break compatibility with old tors by
rejecting RSA1024 and DH1024.

I hope that the debian clang maintainers will look at debian bug
903709 soon. But until they do, this should keep our users and our
CI happy on sid with clang.

Closes ticket 26779.

If we're building for Windows, we want to use windows threads no
matter what, and we don't want to link a pthread library even if it
is present.  Fixes bug 27081; bugfix on 1790dc67 in 0.1.0.1-rc.

Conditionalize the pragma that temporarily disables
-Wunused-const-variable.  Some versions of gcc don't support it.  We
need to do this because of an apparent bug in some libzstd headers.
Fixes bug 26785; bugfix on 0.3.2.11.

Work around two different bugs in the OS X 10.10 and later SDKs that would
prevent us from successfully targeting earlier versions of OS X.

Fixes bug 26876; bugfix on 0.3.3.1-alpha.

Closes ticket 20424.

Closes ticket 26372.

These appear to work for me.

We need this trick because some of our Rust tests depend on our C
code, which in turn depend on other native libraries, which thereby
pulls a whole mess of our build system into "cargo test".

To solve this, we add a build script (build.rs) to set most of the
options that we want based on the contents of config.rust.  Some
options can't be set, and need to go to the linker directly: we use
a linker replacement (link_rust.sh) for these.  Both config.rust and
link_rust.sh are generated by autoconf for us.

This patch on its own should enough to make the crypto test build,
but not necessarily enough to make it pass.

This reverts commit 70d91bd0.

This was already added to LIBTOR_A_SOURCES; it doesn't need to get
added again.

Fixes bug 26402. Bugfix on 0.3.4.1-alpha.

Set rustc flags to treat warnings as fatal if configured with
--enable-warnings.