This variable was hard-wired to libdunealugrid.a. This causes errors when
building shared libraries. This patch fixes the issue by removing the variable
and adding the "dunealugrid" target to the LINK_LIBRARIES property of all
targets.

The backup-restore example failed if SIONlib is not found, because the
corresponding read/write methods silently do nothing. This causes the grid to
be restored from an empty stream, resulting in an error.

This patch fixes the issue by restoring the grid from the SIONlib file only if
SIONlib has been found. Otherwise, this option of restoring is silently ignored.