hdf5

Not every system has perl installed in /usr/bin/, so change the shebang
(#!) line to `/usr/bin/env perl` to locate perl on the PATH.

Everything after the first pathname in the shebang line is treated as
a single argument to the command interpreter (/usr/bin/env "perl -w"),
and there is not ordinarily any such program as "perl -w".  So if the
old shebang line used an option such as `-w`, add a `use warnings;`
statement to the script---note that the semantics change slightly.
`bin/destdep` uses a trick to pass `-p` to `/usr/bin/env perl`.  It
couldn't hurt to use the same trick to pass `-w`.

With these changes, `sh autogen.sh` runs on NetBSD.  It ought to still
work on every other system HDF5 supports, too.