cmake: Fix zlib external project to detect if external version is provided
cmake: Fix zlib external project to detect if external version is provided

This commit changes variable zlib_DIR into ZLIB_ROOT. ZLIB_ROOT is the
variable (1) expected by FindZLIB.cmake module provided by CMake
and (2) set in SlicerConfig.cmake.

See https://github.com/Kitware/CMake/blob/v3.5.0/Modules/FindZLIB.cmake#L64-L68

QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi,QIICR/dcmqi