Add generic AMPL interface.

[dpo]

No description provided.

[dpo]

Here is the interface. It builds into the static library, but I haven't yet been able to test it with ampl.jl because I'm not sure how to get it into the shared library.

[dpo]

Is there a reason why compilation isn't occurring in c99 mode?

/home/travis/build/ampl/mp/src/asl/aslinterface.c:111:3: error: ‘for’ loop initial declarations are only allowed in C99 mode

[vitaut]

I've merged this in with minor changes to fix errors when compiling without c99, better type safety and compatibility with C++.

As for the c99 mode, I guess it's just not the default on that compiler version. I don't add -std=c99 or anything like that as ASL doesn't require it. Anyway, I fixed it by compiling in C++ mode which also gives a better type safety.

I'll update the CMake config to include this into the shared library once I fix the master branch which is broken at the moment due to some unrelated refactoring.

Thanks!