Uses the value of the SPOT_SATSOLVER environment variable
to decide how to call the SAT solver.

* src/misc/satsolver.cc, src/misc/satsolver.hh: New files.
* src/misc/Makefile.am: Add them.
* src/tgbaalgos/dtbasat.cc, src/tgbaalgos/dtgbasat.cc: Use satsolver().