Check if DlsMat type is defined (fixes #2)

DlsMat type is defined in SUNDIALS/CVODE versions >= 2.4.0. We use this
type in solve(), and therefore compiling Astrochem with an version of
SUNDIALS that is older than 2.4.0 causes an error. We check if this type
is defined with the configure script, and display an error message if
not.