- Do not use >= clang-3.5, it uses gcc-4.9 stdlibc++ headers and fails
  with standard C++14.
- Tries to support macosx.

This patch avoids the use of std::map<std::string, lp::parameter> to
start the solver/optimizer. Now, parameters are stored in context and
can be parse with lp::context::parse function.

- Remove duplicated constraints.
- Merge inequalities with same function elements.
- add is_valid_solution to check if a solution from solver (that modify
  constraints) is valid.