Added derivative of eigenvalues of symmetric rank2.  Fixes #3563

Made a private "syev" function that gets called by both the
eigenvalue and derivative routines.  Also the compiler shouldn't
complain about variable-length arrays.
Merge pull request #3576 from jwpeterson/deriv_eigen_3563

Added derivative of eigenvalues of symmetric rank2.  Fixes #3563
Merge commit '7bc27b372b76efe5966f63ce46005c3fbacb3a9c'