When we print features, we make an effort to support all the way back to
pre-3.0. However, in the code for versions from 3 onward we call
`FEATURE_SUMMARY` with multiple kinds of elements to print in the same line.
This is only supported in CMake 3.1 and later, making the rather popular CMake
3.0.2 unable to build the library.

Use a single kind of element per invocation. This means we need to provide a
"description" text, which CMake provides for us if provide multiple kinds of
elements.

Make sure to include 'openssl' as a dep when building statically with SHA1DC

We might want OpenSSL to be the implementation for SHA-1 and/or TLS. If we only
want it for TLS (e.g. we're building with the collision-detecting SHA-1
implementation) then we did not indicate this to the systems including us a
static library.

Add OpenSSL to the list also during the TLS decision to make sure we say we
should link to it if we use it for TLS.

It is indeed a list of dependencies for those which include the static archive.
This is in preparation for adding two possible places where we might add openssl
as a dependency.

cmake: move the rule to find static archives close to building clar

If we're building static libraries, we want to use that for building our clar
binary. This is done in 49551254 (2017-09-22; cmake: use static dependencies
when building static libgit2) but that commit included the rule too early,
making it affect the search for iconv, meaning we did not find it when we were
building a static libgit2.

Move the rule to just before building clar, after we've included the rules for
building the library itself. This lets us find and link to the dynamic libiconv.

cmake: allow explicitly choosing SHA1 backend

Right now, if SHA1DC is disabled, the SHA1 backend is mostly chosen
based on which system libgit2 is being compiled on and which libraries
have been found. To give developers and distributions more choice,
enable them to request specific backends by passing in a
`-DSHA1_BACKEND=<BACKEND>` option instead. This completely replaces the
previous auto-selection.

Upstream git.git has changed their default SHA1 implementation to the
collision-detection algorithm SHA1DC in commit e6b07da27 (Makefile: make
DC_SHA1 the default, 2017-03-17). To match upstream, align ourselves and
switch over to SHA1DC by default.

Execute stale tests