ENH: Add ability to compile against zlib-ng

This MR makes some small adjustments to allow znzlib to be compiled against either zlib or zlib-ng. This accompanies changes to the fsl-base project, which can be found at base!138 (merged).

This change only affects the internal gzipPlugin module, so is an ABI compatible change (i.e. projects which depend on znzlib do not need to be recompiled).