Remove C++ subgame-perfect equilibrium solver

[tturocy]

This implements #637. It also includes some minor refactoring of the command-line tool drivers which it was convenient to do while making this change in those files.

[tturocy]

@rahulsavani for you to check please that everything seems to work for you with the affected tools.

[rahulsavani]

I tested gambit-enumpure, gambit-lp, and gambit-lcp on master and on this branch.

I can confirm that the -P option is now gone and all three seem to work fine now, so this PR looks good.

I will note however, that with all three CLI tools, I could not get them to work with the -P option on current master. I would always get a "Error: Dereferenced null pointer", as in:

gambit-enumpure -P sample_extensive_game.efg
Search for Nash equilibria in pure strategies
Gambit version 16.4.0, Copyright (C) 1994-2025, The Gambit Project
This is free software, distributed under the GNU GPL

Error: Dereferenced null pointer

Not our problem any more though once this is merged in I suppose ;-)

[tturocy]

I tested gambit-enumpure, gambit-lp, and gambit-lcp on master and on this branch.

I can confirm that the -P option is now gone and all three seem to work fine now, so this PR looks good.

I will note however, that with all three CLI tools, I could not get them to work with the -P option on current master. I would always get a "Error: Dereferenced null pointer", as in:

gambit-enumpure -P sample_extensive_game.efg
Search for Nash equilibria in pure strategies
Gambit version 16.4.0, Copyright (C) 1994-2025, The Gambit Project
This is free software, distributed under the GNU GPL

Error: Dereferenced null pointer

Not our problem any more though once this is merged in I suppose ;-)

But it is! Now see #647.