add asio as module

[bisegni]

in this way this project can be used directly as submodule without problem and without calling the ./prepare.sh script

[greensky00]

Hi @bisegni , thanks for submitting this PR.

I don't want to break the existing building scripts after merging this change. Since a submodule creates an empty directory in advance, prepare.sh may not work. So can you modify this line as follows?

if [ ! -d asio/asio ]; then

And please use the official release commit, not the current master. The latest release of Asio is: https://github.com/chriskohlhoff/asio/commit/147f7225a96d45a2807a64e443177f621844e51c

And also, please add a description of how to set up the submodule in Section 2 of README file.

$ git submodule update --init

Please let me know if you want me to make the above changes.

[bisegni]

Hi @greensky00 I should have fixed and applied all your requests. Let me know

[bisegni]

thank you too or the great project