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.

By default, we want to look at the crates directory of the
submodule, not the toplevel of the submodule.  Fixes bug 25679;
bugfix on 0.3.3.1-alpha.

Georg Koppen authored Mar 28, 2018 and Nick Mathewson committed Mar 28, 2018

When merging the patches for #25310 the libc version requirement in
`GettingStartedRust.md` and `configure.ac` did not get updated to the
now needed 0.2.39.

Fixes bug 25474; bugfix on 0.3.2.5-alpha.