Add zlib to extern (DO NOT MERGE)

[fingolfin]

This adds zlib 1.2.11 to the GAP git repository, as well as build system modifications to make use of a system installed zlib, with a fallback to ours. I expect that PR #2509 will be rebased on this. As such, this PR likely should not be merged independently. But we can discuss this tomorrow.

[codecov]

Codecov Report

Merging #2516 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2516      +/-   ##
==========================================
- Coverage   74.41%    74.4%   -0.02%     
==========================================
  Files         481      479       -2     
  Lines      242658   237683    -4975     
==========================================
- Hits       180583   176845    -3738     
+ Misses      62075    60838    -1237

Impacted Files	Coverage Δ
src/modules.h	33.33% <0%> (-66.67%)	⬇️
src/gvars.h	66.66% <0%> (-33.34%)	⬇️
src/fibhash.h	66.66% <0%> (-33.34%)	⬇️
src/gasman.h	73.07% <0%> (-20.03%)	⬇️
src/calls.h	76.59% <0%> (-19.15%)	⬇️
lib/files.gi	27.04% <0%> (-17.63%)	⬇️
src/hookintrprtr.h	87.5% <0%> (-12.5%)	⬇️
src/objects.h	88.37% <0%> (-11.63%)	⬇️
src/intobj.h	89.18% <0%> (-10.82%)	⬇️
src/range.h	90% <0%> (-10%)	⬇️
... and 86 more

[ChrisJefferson]

This is subsumed by #2509, so I think can be closed.