If you specify it that way, you have to set
Code:
-DZLIB_LIBRARY=/gpfs/runtime/opt/zlib/1.2.8/lib/libz.so
And check if the "libz.so" is in this folder.
Did you try to set
Code:
-DZLIB_INCLUDE_DIRS=/gpfs/runtime/opt/zlib/1.2.8/
I think this should find both, library and header.
Best,
Christoph