Doesn't build on OSX

[colincsl]

readline is keg-only so it doesn't get installed to the normal system paths in OSX 10.9.

I added /usr/local/opt/readline to the CMAKE_PREFIX path and it worked.

[meyerj]

For future reference: The creation of symlinks at /usr/local can be enforced with

brew link [--force] readline

which is what has been applied for Travis at orocos-toolchain/orocos_toolchain#34, for example. Setting the CMAKE_PREFIX_PATH manually to the location where readline is installed is indeed another option.