add ltlsynt executable

For now, ltlsynt only handles LTL realizability. It uses a reduction to
parity game followed by Calude et al.'s reduction from parity game to
reachability game.

* bin/ltlsynt.cc, bin/Makefile.am, bin/man/ltlsynt.x,
bin/man/Makefile.am, bin/.gitignore: New binary.
* doc/org/arch.tex, doc/Makefile.am, doc/org/tools.org,
doc/org/ltlsynt.org: Document it.
* spot/misc/game.cc, spot/misc/game.hh, spot/misc/Makefile.am: Parity
game wrapper for parity automata + reachability game interface from
Calude et al.'s paper.

doc/org/ltlsynt.org

@@ -0,0 +1,55 @@
+# -*- coding: utf-8 -*-
+#+TITLE: =ltlsynt=
+#+DESCRIPTION: Spot command-line tool for synthesizing AIGER circuits from LTL/PSL formulas.
+#+SETUPFILE: setup.org
+#+HTML_LINK_UP: tools.html
+
+* Basic usage
+
+This tool answers whether a controller can be built given an LTL/PSL formula
+specifying its behavior.  =ltlsynt= is typically called with
+the following three options:
+
+- =--input=: a comma-separated list of input signal names
+- =--output=: a comma-separated list of output signal names
+- =--formula= or =--file=: the LTL/PSL specification.
+
+The following example is unrealizable, because =a= is an input, so no circuit
+can guarantee that it will be true eventually.
+
+#+BEGIN_SRC sh :results verbatim :exports both
+ltlsynt --input=a --output=b -f 'F a'
+#+END_SRC
+
+#+RESULTS:
+#+begin_example
+UNREALIZABLE
+#+end_example
+
+* TLSF
+
+=ltlsynt= was made with the [[http://syntcomp.org/][SYNTCOMP]] competition in
+mind, and more specifically the TLSF track of this competition.  TLSF is a
+high-level specification language created for the purpose of this competition.
+Fortunately, the SYNTCOMP organizers also provide a tool called =syfco= which
+can translate a TLSF specification to an LTL formula.
+
+The following four steps show you how a TLSF specification called spec.tlsf can
+be tested for realizability using =syfco= and =ltlsynt=:
+
+#+BEGIN_SRC sh
+LTL=$(syfco FILE -f ltlxba -m fully)
+IN=$(syfco FILE -f ltlxba -m fully)
+OUT=$(syfco FILE -f ltlxba -m fully)
+ltlsynt --formula="$LTL" --input="$IN" --output="$OUT"
+#+END_SRC
+
+* Algorithm
+
+The tool reduces the synthesis problem to a parity game, and solves the parity
+game using Zielonka's recursive algorithm.  The full reduction from LTL to
+parity game is described in a paper yet to be written and published.
+
+You can ask =ltlsynt= not to solve the game and print it instead (in the
+PGSolver format) using the =--print-pg= option, and leaving you the choice of
+an external solver such as PGSolver.