Autodetect if symlinks are supported on target file system

[techtonik]

This detects symlink suppot on destination filesystem and not only in Python interpreter or OS. If symlinks are not supported, falls back to copy behavior.

This fixes some case reported in #92 and #409, without specifying --always-copy explicitly.

Ideal case would be to invert virtualenv symlinking behavior - symlink only when explicitly requested.

[BrownTruck]

Hello!

As part of an effort to ease the contribution process and adopt a more standard workflow virtualenv has switched to doing development on the master branch. However, this Pull Request was made against the develop branch so it will need to be resubmitted against master.

If you do nothing, this Pull Request will be automatically migrated by @BrownTruck for you.

If you would like to retain control over this pull request then you should resubmit it against the master branch, closing and referencing the original Pull Request.

If you choose to migrate this Pull Request yourself, here is an example message that you can copy and paste:

This detects symlink suppot on destination filesystem and not only in Python interpreter or OS. If symlinks are not supported, falls back to copy behavior.

This fixes some case reported in #92 and #409, without specifying --always-copy explicitly.

Ideal case would be to invert virtualenv symlinking behavior - symlink only when explicitly requested.

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*This was migrated from pypa/virtualenv#663 to reparent it to the ``master`` branch. Please see original pull request for any previous discussion.*

If this pull request is no longer needed, please feel free to close it.