Switched to InstallSymlink for creating symlinks

OpenRAVE previously created 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 macro, from [1], which properly handles this case. This
script is part of Bro [2] and, thus, is is covered by a 3-clause BSD
license [3].

[1] https://github.com/bro/cmake/blob/master/InstallSymlink.cmake
[2] http://www.bro.org/
[3] https://github.com/bro/bro/blob/master/COPYING
Merge pull request #304 from personalrobotics/cmake-symlink

Switched to InstallSymlink for creating symlinks