Include https://github.com/ethomson/ntlmclient as a dependency.

Our PCRE dependency has uncommented fallthroughs in switch statements.
Turn off warnings for those in the PCRE code.

PCRE includes compatibility functions that may go unused.  Don't warn.

Use PCRE 8.42 as the builtin regex implementation, using its POSIX
compatibility layer.  PCRE uses ASCII by default and the users locale
will not influence its behavior, so its `regcomp` implementation is
similar to `regcomp_l` with a C locale.

It seems like MingW64's size_t is defined differently than in Linux.

While our contribution guide tries to make clear the licenses that apply
to libgit2, it does not make clear that different licenses apply to our
bundled dependencies. Make this clear by listing each dependency
together with the licenses that they are governed by. Furthermore,
bundle the complete license texts next to the code they apply to.

GCC 7 has introduced new warnings for implicit fallthrough in switch
statements. Whenever there is no branch in a case block, GCC will watch
out for some heuristics which indicate that the implicit fallthrough is
intended, like a "fallthrough" comment. The third-party http-parser code
manages to trick this heuristic in one case, even though there is a
"FALLTHROUGH" comment. Fortunately, GCC has also added a strictness
level to the -Wimplicit-fallthrough diagnostic, such that we can loosen
this heuristic and make it more lax.

Set -Wimplicit-fallthrough=1 in http-parser's CMake build instructions,
which is the strictest level that gets rid of the warning. This level
will treat any kind of comment as a "fallthrough" comment, which
silences the warning.

The current version of zlib bundled with libgit2 is version 1.2.8. This
version has several CVEs assigned:

- CVE-2016-9843
- CVE-2016-9841
- CVE-2016-9842
- CVE-2016-9840

Upgrade the bundled version to the current release 1.2.11, which has
these vulnerabilities fixes.

Include the constants for `WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_1` and
`WINHTTP_FLAG_SECURE_PROTOCOL_TLS1_2` so that they can be used by mingw.

This updates both the `deps/winhttp` framework (for classic mingw) and
adds the defines for mingw64, which does not use that framework.

Due to our split of CMake files into multiple modules, we had to replace
some uses of the `${CMAKE_CURRENT_SOURCE_DIR}` and
`${CMAKE_CURRENT_BINARY_DIR}` variables and replace them with
`${CMAKE_SOURCE_DIR}` and `${CMAKE_BINARY_DIR}`. This enabled us to
still be able to refer to top-level files when defining build
instructions inside of a subdirectory.

When replacing all variables, it was assumed that the absolute set of
variables is always relative to the current project. But in fact, this
is not the case, as these variables always point to the source and
binary directory as given by the top-levl project. So the change
actually broke the ability to include libgit2 directly as a subproject,
as source files cannot be found anymore.

Fix this by instead using project-specific source and binary directories
with `${libgit2_SOURCE_DIR}` and `${libgit2_BINARY_DIR}`.