…ed libgit2 (#822)

Since libgit2/libgit2@3a3ab06 libgit2 is refactored into a library and CLI. This change allows git2-rs to build those sources when the  copy is used.

This should not be integrated until a libgit2 release is made incorporating those sources. The libgit2 submodule has been updated at the current commit on libgit2's `main` branch.

libgit2 does not have a stable ABI across minor versions, as has been
demonstrated in the last few minor releases, see #813 and #746. This
pain is primarily suffered by users of rolling release distros, because
they tend to get a new libgit2 version before the authors of libraries
release versions that pull in the new libgit2-sys version which works
with the new binary.

This patch means that going forward, users don't need to rush to
upgrade their version of libgit2-sys or suffer errors/UB in the
meantime. If the system version changes, they will just start using
the vendored version which has been tested against the bindings.

In its default configuration, pkg-config adds system-wide library
directories to the linker search path (rust-lang/pkg-config-rs#11).
This causes these directories to be searched before other paths added
by later crates or by -Clink-arg in rustflags. If a library is present in
the system-wide directory and a later build step wants to specifically
link against a different version of that library in another path, the linker
will choose the library from the first search directory it finds. If the linker
doesn't find a library in any of the specified search directories, it falls
back on system-wide paths, so we don't need to print the
path we found libssh2 in if it is in one of those system paths.

See rust-lang/libz-sys#50 for the same fix to libz that landed a while back.
See also alexcrichton/curl-rust#441 for the same fix to libcurl-sys.

…at (#833)

Use a weak dependency feature to enable zlib-ng-compat on libssh2-sys
without forcibly enabling libssh2-sys.