Introduce `check_prototype_definition_safe` that is safe for `Werror`
usage.

xdiff: move xdiff to 'deps'

Add support for SSPI on Windows, which offers NTLM and Negotiate
authentication.

Provide a stream interface for Schannel - the native crypto APIs - on
Windows. This allows Windows to use the same HTTP transport that all the
other platforms use, with its own native crypto.

Ultimately this allows us to deprecate WinHTTP and we need not add
support for our socket changes in two places (our HTTP stack and the
WinHTTP stack).

xdiff is a dependency (from git core) and more properly belongs in the
'deps' directory. Move it there, and add a stub for cmake to resolve
xdiff from the system location in the future. (At present, bundled xdiff
remains hardcoded.)

Not everybody builds libgit2 using cmake; provide an `experimental.h`
with no experiments configured for those that do not. To support this,
we also now create compile definitions for experimental functionality,
to supplant that empty `experimental.h`. cmake will continue to generate
the proper `experimental.h` file for use with `make install`.

RFC: SHA256 proof of concept

since f585b12 neither PCRE nor PCRE2 backends rely on POSIX regex, so
reflect it in library detection logic

libgit2 can be built with optional, experimental sha256 support. This
allows consumers to begin testing and providing feedback for our sha256
support while we continue to develop it, and allows us to make API
breaking changes while we iterate on a final sha256 implementation.

The results will be `git2-experimental.dll` and installed as
`git2-experimental.h` to avoid confusion with a production libgit2.