Make `lbool` explicitly signed

This avoids issues due to some platforms making `char` signed and others
unsigned. For example, currently the result of promoting `(lbool)-1` to `int`
can differ on different platforms. See
50ffa10848/lib/bill/bill/sat/interface/abc_bsat2.hpp (L156)
for an example of that.

src/sat/bsat/satVec.h

@@ -130,7 +130,9 @@ static inline void   vecp_remove(vecp* v, void* e)
 typedef int    lit;
 typedef int    cla;
 
-typedef char               lbool;
+// Explicitly make it signed so promotion-to-int behavior doesn't vary
+// across platforms that define signedness of char differently.
+typedef signed char lbool;
 
 static const int   var_Undef = -1;
 static const lit   lit_Undef = -2;