Switched to InstallSymlink for creating symlinks

OpenRAVE currently creates symlinks with an install(CODE ...) directive during the install step. Symlinks created in this way are not packaged correctly when debhelper to build a Debian package. I switched to the
[InstallSymlink](https://github.com/bro/cmake/blob/master/InstallSymlink.cmake) macro which properly handles this case. This script is part of [Bro](http://www.bro.org/) and, thus, is is covered by a [3-clause BSD license](https://github.com/bro/bro/blob/master/COPYING).