Make lowdown optional

[wegank]

Motivation

This PR makes the lowdown library optional. Specifically, when --disable-markdown is passed, renderMarkdownToTerminal becomes trivial, i.e. Markdown is not rendered:

$ nix help

> **Warning** \
> This program is
> [**experimental**](@docroot@/contributing/experimental-features.md#xp-feature-nix-command)
> and its interface is subject to change.

# Name

`nix` - a tool for reproducible and declarative configuration management
...

Context

lowdown currently has no MinGW support (kristapsdz/lowdown#130). While this PR is inspired by the commit with the same name in #4953, the latter hasn't been updated for two years. I think it's good to have a PR with an atomic commit to drive the changes forward, one by one.

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[Ericson2314]

Thanks @wegank! Can you combine with a3d71fb from #8901?

[wegank]

I don't think readline support has regressed since de59973? Like, it's still possible to build Nix on Linux with

  env = {
    EDITLINE_LIBS = "${readline}/lib/libhistory${stdenv.hostPlatform.extensions.sharedLibrary} ${readline}/lib/libreadline${stdenv.hostPlatform.extensions.sharedLibrary}";
    EDITLINE_CFLAGS = "-DREADLINE";
  };

So I didn't try to make editline optional...

[Ericson2314]

OK fair enough :) ignore readline/editline for now is fine. But the "yes vs auto" I still had in my configure.ac I think is still good?

You should also do a package.nix update so we are passing the new flag; see how the other flags are done at the Nix level. The flag can be default with hostPlatform.isWindows.

[Ericson2314]

Thanks!

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-on-windows/1113/99