First stab at collection of CMakeLists.txt for Boost.GIL
- Allow building and testing boostorg/gil against Boost from
cloned superproject or installed distribution.
Add CMakeSettings.json config file for VS2017 integration with CMake
- Defines build configurations for VS and Ninja generators.
- Can be used as is or as a template ready to customise.
Add conanfile.txt for Conan package manager (eg. for cmake -DGIL_USE_CONAN=ON).
Add .editignore file to with basic encoding of CMake and CI scripts.
Update .appveyor.yml with two extra CMake-based builds (allowed to fail).
