Making the repo suitable for having it as submodule

[alphamarket]

The way the library has been implemented we need to install the library in order to use it, otherwise nothing will work as in the source code you include the files from system's file like `#include <gzip/...> and it made the project impossible for having it as a submodule; since this project is a header-only, the changes in this commit are compatible with the submodule approach and current make it installed approach. Test units are also passing.

[int-ua]

Is there an example on how to use it as a submodule? I assume you meant CMake submodule? I'm also planning to use the repository as a git submodule and looks like this change will make it easier too. CleverRaven/Cataclysm-DDA#44218

[alphamarket]

git submodule add https://github.com/mapbox/gzip-hpp.git gzip

In the project you just include the stuff and start working.

#include "gzip/include/utils.hpp"
#include "gzip/include/compress.hpp"
#include "gzip/include/decompress.hpp"

If you check the changes in the commit the way you are including your local files don't allow to use it as a submodule.

[themightyoarfish]

It's a bit weird having to explicitly refer to the include/ subdirectory though, normally I believe you would either have another layer aka include/gzip/ in the file tree, or install the project inside your build folder when including it from the parent project.