(1) Makefile: added a shared library target, (2) no longer compile the main function as part of libabc.a

xSAT is an experimental SAT Solver based on Glucose v3(see Glucose copyrights below) and ABC C version of
MiniSat (bsat) developed by Niklas Sorensson and modified by Alan Mishchenko. It’s development has reached
sufficient maturity to be committed in ABC, but still in a beta state.

TODO:
* Read compressed CNF files.
* Study the use of floating point for variables and clauses activity.
* Better documentation.
* Improve verbose messages.
* Expose parameters for tuning.

from V. Callegaro, F. S. Marranghello, M. G. A. Martins, R. P. Ribas and A. I. Reis,
entitled "Bottom-up disjoint-support decomposition based on cofactor and boolean difference analysis" presented in ICCD'15.