FindIPOPT.cmake: fix libraries.

This may also solve #153 by linking with Ipopt's dependencies, i.e.
the libraries described in your ipopt.pc (possibly COIN HSL, BLAS,
Lapack, etc.).
ipopt: fix hessian approximation error.

The error was:

Option "hessian_approximation" is not chosen as "limited-memory", but eval_h returns false.
Exception of type: OPTION_INVALID in file "../../../../Ipopt/src/Interfaces/IpTNLPAdapter.cpp" at line 1217:
 Exception message: eval_h is called but has not been implemented