Update find_package(Simbody) mechanism.

Simbody uses a new mechanism for client projects to find Simbody. Also,
Simbody's installation was reorganized on non-WIN32 platforms. To address both
of these, I previously changed OpenSim's find_package(Simbody ...) mechansim to
operate differently depending on the platform (WIN32 or otherwise). In this
commit, I remove the platform-dependent behavior, and include an error message
for if Simbody is not found.