Work around lack of constexpr constructors for Jet

https://github.com/ceres-solver/ceres-solver/issues/965

Change-Id: Ia74a64568815605586c1f326a55cc36b09f33f30

include/ceres/rotation.h

diff --git a/include/ceres/rotation.h b/include/ceres/rotation.h
index a518ec8..b7061c5 100644
--- a/include/ceres/rotation.h
+++ b/include/ceres/rotation.h
@@ -610,8 +610,8 @@
   //   [-pi, pi) x [0, pi / 2) x [-pi, pi)
   // which is enforced here
   if constexpr (EulerSystem::kIsProperEuler) {
-    constexpr T kPi = constants::pi_v<T>;
-    constexpr T kTwoPi(2.0 * kPi);
+    const T kPi(constants::pi_v<T>);
+    const T kTwoPi(2.0 * kPi);
     if (euler[1] < T(0.0) || ea[1] > kPi) {
       euler[0] += kPi;
       euler[1] = -euler[1];