Merge branch 'ps/zlib-ng'

The code paths to interact with zlib has been cleaned up in
preparation for building with zlib-ng.

* ps/zlib-ng:
  ci: make "linux-musl" job use zlib-ng
  ci: switch linux-musl to use Meson
  compat/zlib: allow use of zlib-ng as backend
  git-zlib: cast away potential constness of `next_in` pointer
  compat/zlib: provide stubs for `deflateSetHeader()`
  compat/zlib: provide `deflateBound()` shim centrally
  git-compat-util: move include of "compat/zlib.h" into "git-zlib.h"
  compat: introduce new "zlib.h" header
  git-compat-util: drop `z_const` define
  compat: drop `uncompress2()` compatibility shim

.gitlab-ci.yml

@@ -60,7 +60,7 @@ test:linux:
         CC: clang
       - jobname: pedantic
         image: fedora:latest
-      - jobname: linux-musl
+      - jobname: linux-musl-meson
         image: alpine:latest
       - jobname: linux32
         image: i386/ubuntu:20.04