Don't rely on constexpr lambdas

This is to appease clang 3.8

src/linearsolver.hh

@@ -75,7 +75,7 @@ namespace PPS {
     //! apply operator to x:  \f$ y = A(x) \f$
     void apply (const X& x, Y& y) const override
     {
-      static constexpr auto id = [](auto v){ return v; };
+      auto id = [](auto v){ return v; };
       y = 0;
       go_->nonlinear_jacobian_apply(linearizationPoint_, x, y, id);
     }