Co-authored-by: Edward Thomson <ethomson@github.com>

Provide an interface around OpenSSL to dynamically load the libraries
and symbols, so that users can distribute a libgit2 library that is not
linked directly against OpenSSL.  This enables users to target multiple
distributions with a single binary.

This mechanism is optional and disabled by default.  Configure cmake
with -DUSE_HTTPS=OpenSSL-Dynamic to use it.

In the past, we've imported the CheckPrototypeDefinition into our own
module directory as it wasn't yet available in all supported CMake
versions. Now that we require at least CMake v3.5, we don't need to
bundle it anymore as it's included with the distribution already.

Let's drop the included modules and always use upstream's version.

Our custom CMake module currently live in "cmake/Modules". As the
"cmake/" directory doesn't contain anything except the "Modules"
directory, it doesn't really make sense to have the additional
intermediate directory. So let's instead move the modules one level up
into the "cmake/" top level directory.

We currently use `FILE(GLOB ...)` in most places to find source and
header files. This is problematic in that the order of files returned
depends on the operating system's directory iteration order and may thus
not be deterministic. As a result, we link object files in unspecified
order, which may cause the linker to emit different code across runs.

Fix this issue by sorting all code used as input to the libgit2 library
to improve the reliability of reproducible builds.

We're currently doing unnecessary work to auto-detect backends even if
the functionality is disabled altogether. Let's fix this by removing the
extraneous FOO_BACKEND variables, instead letting auto-detection modify
the variable itself.

We currently hand-code logic to configure where to install our artifacts
via the `LIB_INSTALL_DIR`, `INCLUDE_INSTALL_DIR` and `BIN_INSTALL_DIR`
variables. This is reinventing the wheel, as CMake already provide a way
to do that via `CMAKE_INSTALL_<DIR>` paths, e.g. `CMAKE_INSTALL_LIB`.
This requires users of libgit2 to know about the discrepancy and will
require special hacks for any build systems that handle these variables
in an automated way. One such example is Gentoo Linux, which sets up
these paths in both the cmake and cmake-utils eclass.

So let's stop doing that: the GNUInstallDirs module handles it in a
better way for us, especially so as the actual values are dependent on
CMAKE_INSTALL_PREFIX. This commit removes our own set of variables and
instead refers users to use the standard ones.

As a second benefit, this commit also fixes our pkgconfig generation to
use the GNUInstallDirs module. We had a bug there where we ignored the
CMAKE_INSTALL_PREFIX when configuring the libdir and includedir keys, so
if libdir was set to "lib64", then libdir would be an invalid path. With
GNUInstallDirs, we can now use `CMAKE_INSTALL_FULL_LIBDIR`, which
handles the prefix for us.