Fix Eigen >= 3.3 compilation if EIGEN_DONT_VECTORIZE set

Would result in compilation error

include/ceres/jet.h:296:59: error: requested alignment is not a positive power of 2
   alignas(kAlignment) Eigen::Matrix<T, N, 1, kAlignOrNot> v;

because CERES_JET_ALIGN_BYTES = EIGEN_MAX_ALIGN_BYTES = 0 in that case.

Change-Id: I247653d3b7555e80891d3fb78676acfa302a9e1d

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index ff03f68..b711ea2 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -258,7 +258,7 @@
   // supported is < 16, in which case we do not specify an alignment, as this
   // implies the host is not a modern x86 machine.  If using < C++11, we cannot
   // specify alignment.
-#ifndef CERES_USE_CXX11
+#if !defined(CERES_USE_CXX11) || defined(EIGEN_DONT_VECTORIZE)
   // Without >= C++11, we cannot specify the alignment so fall back to safe,
   // unvectorised version.
   Eigen::Matrix<T, N, 1, Eigen::DontAlign> v;