Add zlib-ng 2.1.6, remove zlib

[mathomp4]

Closes #15

This PR moves ESMA-Baselibs from zlib (which is stable, but might have issues with newer hardware, etc.) to zlib-ng which is an actively maintained and developed library which seems to provide better performance.

Tests show this is zero-diff for GEOS. It all runs just fine.

Only tested on discover. Need to test on macOS.

[mathomp4]

This fails on macOS. Or, rather, it builds just fine on macOS, but nothing can link to it. For example with curl:

configure: error: one or more libs available at link-time are not available run-time. Libs used at link-time: -lz -lm

and from curl/config.log:

configure:34993: $? = 0
configure:34993: ./conftest
dyld[61616]: Library not loaded: @rpath/libz.1.dylib
  Referenced from: <39FAB902-1943-35CE-A6E2-A9ED30F8C08F> /Users/mathomp4/Baselibs/ESMA-Baselibs-7.19.0-TEST/src/curl/conftest
  Reason: no LC_RPATH's found
./configure: line 2529: 61616 Abort trap: 6           ./conftest$ac_exeext
configure:34993: $? = 134
configure: program exited with status 134