Conan to help building on Windows

[Antoine Villeret <antoine.villeret@xxxxxxxxx>]

Hi,

Since I had difficulties to find 64bit binaries of OpenSSL, I decided to
try out Conan (something like a crossplatform package managing system).
The setup is pretty simple, and I was surprised how simple it was to build
libssh with it.
Basically I added a conanfile.txt describing dependencies and I modified
CMakeLists.txt to read the generated .cmake file.

The steps are (from libssh folder) :

    mkdir build && cd build
    conan install ..
    cmake .. -G "Visual Studio 14 Win64"
    cmake --build . --config Release

You can see the changes on my github fork
https://github.com/avilleret/libssh/commit/4e3951110d466f6b69fec5302c8d7c17e203e68a

For now I only added OpenSSL dependency since I found ZLib 64 binary, but
we could certainly add ZLib too.

Hope that helps.

Cheers

Antoine