MNT: Add ability to link against zlib-ng

Paul McCarthyrequested to merge
mnt/zlib-ng into master

This MR adds a new configuration setting which allows FSL projects to be linked against zlib-ng, an accelerated alternative to zlib. This can be done by setting a variable ZNZLIB_USE_ZLIB_NG to any non-empty string value at compilation time.

This MR accompanies changes to znzlib which can be found at znzlib!6 (merged).

At the moment we are using zlib-ng in "dual-linking" mode, where both zlib and zlib-ng can be linked into the same binary. This is so that other third-party libraries (e.g. ciftilib) can continue to use functions from zlib.

We will continue to use zlib for the moment, but having these changes in place will allow us to switch over to zlib-ng in the future.

Our strategy may need to be revisited in the future, as there are tentative plans by the conda-forge team to build zlib-ng as a variant/drop-in replacement of zlib: https://github.com/conda-forge/conda-forge.github.io/issues/2638

Edited by Paul McCarthy

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