Fixed error that prevents compilation if Z3 is not present.

Former-commit-id: d8de79f2ae

src/solver/Z3SmtSolver.cpp

@@ -46,11 +46,13 @@ namespace storm {
         : context(nullptr), solver(nullptr), expressionAdapter(nullptr), lastCheckAssumptions(false), lastResult(CheckResult::Unknown)
 #endif
 		{
+#ifdef STORM_HAVE_Z3
             z3::config config;
             config.set("model", true);
             context = std::unique_ptr<z3::context>(new z3::context(config));
             solver = std::unique_ptr<z3::solver>(new z3::solver(*context));
             expressionAdapter = std::unique_ptr<storm::adapters::Z3ExpressionAdapter>(new storm::adapters::Z3ExpressionAdapter(*context, true));
+#endif
         }
         
 		Z3SmtSolver::~Z3SmtSolver() {