...whose (static) library is called "zlib" instead of just "z".

(I ran into this when trying to do a 32-bit Linux build in a 64-bit environment,
with only very limited 32-bit support installed in the system.)

Change-Id: I9286975917ddf643a22803561677af035e66fb98
Reviewed-on: https://gerrit.libreoffice.org/28964Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>