Bugfix: step-53 requires enabled DEAL_II_WITH_ZLIB

step-53 uses boost's <iostreams/filter/gzip.hpp> in order to unpack the
file topography.txt.gz.

If the internal boost variant is used this requires external linkage to
zlib.
Merge pull request #132 from tamiko/fix_step53

Bugfix: step-53 requires enabled DEAL_II_WITH_ZLIB