VTK classes that define filenames at compile time do not specify a
consistent length. This patch uses the logic in kwsys to define
VTK_MAXPATH. A number of files have been identified that specify a
variety of maximum path lengths. This defined constant should be used
in future classes that allocate filenames at complie time.

Maximum path lengths are not well defined in general. Even though some
systems define a maximum, they may still allow longer path names. The
purpose here is to provide a consistent value for VTK. However, this
is no less flawed than than current practices.