configure fix

[mattdowle]

Follow up to #3984 which renamed Makevars to Makevars.in and now requires the end of configure to run
Closes #4640
Closes #4650
Closes #4652

removed the exit 0
put the zlib version output into an else (that was why the exit 0 was there because there was nothing after the zlib version output originally, i.e. before #3984)
added the -- for FreeBSD, thanks to @wesnm
changed wording for Mac to explain that zlib is built in there
news item

[codecov]

Codecov Report

Merging #4662 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4662   +/-   ##
=======================================
  Coverage   99.61%   99.61%           
=======================================
  Files          73       73           
  Lines       14244    14244           
=======================================
  Hits        14189    14189           
  Misses         55       55           

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