[meson] Change Zlib configuration option.

* meson_options.txt, meson.build: Change the format of the 'zlib' meson
build configuration option to be a combo with the following choices:

  - none: Do not support gzip-compressed streams at all.

  - internal: Support gzip-compressed streams using the copy of the gzip
    sources under `src/gzip/`; this should only be used during development
    to ensure these work properly.

  - external: Support gzip-compressed streams using the 'zlib' Meson
    subproject, linked as a static library.

  - system: Support gzip-compressed streams using a system-installed version
    of zlib.

  - auto: Support gzip-compressed streams using a system-installed version
    of zlib, if available, or using the 'zlib' subproject otherwise.  This
    is the default.

  - disabled: Backward-compatible alias for 'none'.

  - enabled: Backward-compatible alias for 'auto'.
2 files changed