Fixes #149. Change LIBDIR if installing to /usr/local.

Made corresponding changes to the build/install instructions.

Tested with 3 different `CMAKE_INSTALL_PREFIX`'s:

1. /usr/local
2. /home/chris/simbody
3. /usr

Works as intended (does not create x86_64-linux-gnu if installing to /usr/local). In the first case, I had to run `sudo ldconfig` to get an example executable to find the libraries (in `/usr/local/lib`), which I did not anticipate. Maybe a system restart would have done this for me, but simply opening a new terminal window was not sufficient. In the third case, running `ldconfig` was not necessary.

We still may not want to do this. It adds a special case to the installation, and thus adds complexity. However, I think people should not have to set `LD_LIBRARY_PATH` if they install into a default location.