fixed illegal friend declaration of mkLiteral (same issue in minisat)

According to ISO C++11 standard:
"If a friend declaration specifies a default argument expression, that
declaration shall be a definition and shall be the only declaration of the
function or function template in the translation unit.”

Solved in the same way as the minisat project niklasso/minisat#17:
niklasso/minisat#17

src/solver_types.hh

@@ -49,15 +49,15 @@ struct Literal {
     int x;
 
     // Use this as a constructor:
-    friend Literal mkLiteral(Variable var, bool sign = false);
+    //friend Literal mkLiteral(Variable var, bool sign = false);
 
     bool operator == (Literal p) const { return x == p.x; }
     bool operator != (Literal p) const { return x != p.x; }
     bool operator <  (Literal p) const { return x < p.x;  } // '<' makes p, ~p adjacent in the ordering.
     bool operator <= (Literal p) const { return x <= p.x;  }
 };
 
-inline  Literal  mkLiteral     (Variable var, bool sign) { Literal p; p.x = var + var + (int)sign; return p; }
+inline  Literal  mkLiteral (Variable var, bool sign = false) { Literal p; p.x = var + var + (int)sign; return p; }
 inline  Literal  operator ~(Literal p)              { Literal q; q.x = p.x ^ 1; return q; }
 inline  Literal  operator ^(Literal p, bool b)      { Literal q; q.x = p.x ^ (unsigned int)b; return q; }
 inline  bool sign      (Literal p)              { return p.x & 1; }