Add export from buildtree

[VestniK]

Allows to use library without installing it.

[podsvirov]

Hello @VestniK, thanks for PR and use case, but...
In my experience export(EXPORT...) is a generation time command. If we add asio via add_subdirectory the asio-targets.cmake file will not we available immediate and we could not link our targets to asio::standalone at first fime. What your think about this?

[VestniK]

My main use case for build tree export is opening several project in one IDE session and make make them find each others build trees. It allows to build all projects at once and debug code across several projects. This setup is very flexible. I can open only few projects from the whole depndencies graph and get rest of them from the install trees.

[podsvirov]

@VestniK, merged to target branch.
I also apply this idea to upstream PR chriskohlhoff#128.
Thank you.