[zlib] Ignore external ZLIB_DLL

[dg0yt]

• What does your PR fix?

  Hardcodes the triplet linkage into the installed headers, so that usage no longer depends on external definition of ZLIB_DLL. For example, GDAL always applies ZLIB_DLL. Noticed in log from https://github.com/microsoft/vcpkg/pull/26676#issuecomment-1251764155:

  LINK : warning LNK4286: symbol 'crc32' defined in 'zlib.lib(crc32.obj)' is imported by 'gdal.lib(cpl_minizip_unzip.cpp.obj)'
  LINK : warning LNK4217: symbol 'deflateInit_' defined in 'zlib.lib(deflate.obj)' is imported by 'gdal.lib(cpl_vsil_gzip.cpp.obj)' in function 'CPLZLibDeflate'
  

  Given the broad impact of port zlib changes, this PR should be considered for combination with other zlib changes: #26792

• Which triplets are supported/not supported? Have you updated the CI baseline?

  unchanged, no

• Does your PR follow the maintainer guide?

  yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  yes

[Neumann-A]

Why has this not been a problem before?

[dg0yt]

Why has this not been a problem before?

a) Packages tend to unconditionally set ZLIB_DLL. Evidence: gdal as mentioned aboved; git grep ZLIB_DLL b) There is only a warning in static builds, not an error. c) Contributors and reviewers don't care for this warning. In particular when connected with other errors Evidence: the mentioned gdal issue; https://github.com/microsoft/vcpkg/search?q=LNK4217&type=issues

[dg0yt]

libkml probably stumbles over a mix of headers from zlib and its vendored copy of minizip.

[dg0yt]

I want to improve the libkml mingw patch.

[dg0yt]

Please do the cherry-pick.

[BillyONeal]

Thanks again!