Include additional headers during cmake install

[pabiswas]

cmake install only copies date/date.h in the installation path.
This PR adds the other headers in the installation path during cmake install.

[basilgello]

This is useful change and it closely resembles the one Debian uses and I am using in Kodi (yet this change might need CMake older than 3.15). I am now going to check if this PR works as expected with current CMake and CMake < 3.15 before I mark it as ready.

[basilgello]

Great! Although not principal, the changed line should be indented for better readability. I dont know why Android client app for Github that I use has chewed leading spaces :(

[basilgello]

@HowardHinnant can be merged with indentation fix :)

[HowardHinnant]

I'm not comfortable making this change. It isn't that I don't like the change. It is that I don't understand the git process well enough to be assured I will make the change without messing it up.

[basilgello]

Moment, let me do the branch off it and show how it should look like!

[basilgello]

git checkout master
git pull
git fetch https://github.com/basilgello/date include-headers
git merge --ff FETCH_HEAD

Branch: https://github.com/basilgello/date/commits/include-headers

[HowardHinnant]

Thanks! Merged: 3613526