docs: Update dependencies list in README.

  - Remove unused dependencies: capnproto and sqlite.

  - Restructure Windows section to highlight that the CNG backend is
    recommended on Windows.

  - Fix MSYS2 instructions to use `pacman` instead of `pacboy`.

  - Add mention of 32-bit build on Windows.

  - Remove Brew instructions that apparently are no longer needed in
    latest versions of Brew.

  - Test all variants. Windows and MacOS were tested in VMs and *nix
    distributions were tested in respective Docker containers. One issue
    worth mentioning that the NixOS script had to be run on version
    2.3.16 as their later images seem to suffer from the regression of
    issue #137982 [0] (tested thanks to @puzzlewolf).

  - Fix #673 as now the minimal set is valid for both the openpgp crate
    and others (there are no additional dependencies). The only
    exception is the OpenSSL package required on Debian and Fedora. This
    has been explicitly clarified in the `Notes` section.

  - Fix #785.

[0]: https://github.com/NixOS/nixpkgs/issues/137982

1 files changed, 45 insertions, 28 deletions

diff --git a/README.md b/README.md
index 12aa71ed..7c6f91f4 100644
--- a/README.md
+++ b/README.md
@@ -203,7 +203,7 @@ see below for OS-specific commands to install the needed libraries:
 ### Debian
 
 ```shell
-$ sudo apt install git rustc cargo clang libclang-dev make pkg-config nettle-dev libssl-dev capnproto libsqlite3-dev
+# apt install cargo clang git nettle-dev pkg-config libssl-dev
 ```
 
 Notes:
@@ -213,21 +213,28 @@ Notes:
     distribution only includes an older Rust version.
   - You need at least Nettle 3.4.1.  Both the versions in Debian 10 (Buster)
     and Debian 11 (Bullseye) are fine.
+  - `libssl-dev` is only required by the `sequoia-net` crate and
+    crates depending on it (`sq`).
 
 [rustup]: https://rustup.rs/
 
 ### Arch Linux
 
 ```shell
-$ sudo pacman -S git cargo clang make pkg-config nettle openssl capnproto sqlite3 --needed
+# pacman -S clang git pkgconf rustup --needed
 ```
 
 ### Fedora
 
 ```shell
-$ sudo dnf install git rustc cargo clang make pkg-config nettle-devel openssl-devel capnproto sqlite-devel
+# dnf install cargo clang git nettle-devel openssl-devel
 ```
 
+Notes:
+
+  - `openssl-devel` is only required by the `sequoia-net` crate and
+    crates depending on it (`sq`).
+
 ### NixOS
 
 Development environment for use with `nix-shell` or `direnv`:
@@ -247,7 +254,6 @@ pkgs.mkShell {
   buildInputs = [
     nettle
     openssl
-    sqlite
   ];
 
   nativeBuildInputs = [
@@ -257,7 +263,6 @@ pkgs.mkShell {
 
     llvmPackages.clang
     pkgconfig
-    capnproto
 
     # tools
     codespell
@@ -278,19 +283,13 @@ pkgs.mkShell {
 #### MacPorts
 
 ```shell
-$ sudo port install cargo rust capnproto nettle pkgconfig coreutils
+$ sudo port install cargo nettle pkgconfig
 ```
 
 #### Brew
 
 ```shell
-$ brew install rust capnp nettle
-```
-
-If building the transitive dependency `nettle-sys` reports missing `libclang.dylib` file make sure that `DYLD_LIBRARY_PATH` is set correctly:
-
-```shell
-export DYLD_LIBRARY_PATH=/Library/Developer/CommandLineTools/usr/lib/
+$ brew install rust nettle
 ```
 
 ### Windows
@@ -299,25 +298,20 @@ Please make sure to preserve line-endings when cloning the Sequoia
 repository.  The relevant git option is `core.autocrlf` which must be
 set to `false`.
 
-#### MSYS2
-
-You can install the needed libraries with the following command:
-
-```shell
-$ pacboy -S base-devel toolchain:x clang:x bzip2:x nettle:x sqlite3:x capnproto:x
-```
-
-Due to Gitlab's Windows Shared Runners being somewhat slow, we only
-run them automatically for MRs, which contain `windows` in the branch
+Due to Windows Runners being somewhat slow, we only run them
+automatically for MRs, which contain `windows` in the branch
 name. Please name your branch accordingly when contributing a patch
 which might affect Windows.
 
-#### MSVC
+#### CNG
 
-To build Sequoia, you need to have [`capnp`] tool installed.
+On Windows Sequoia PGP can use one of several cryptographic backends.
+The recommended one is Windows Cryptography API (CNG) as it doesn't
+require any additional dependencies.  The standard tooling required to
+build native dependencies ([Visual Studio Build Tools][]) is still
+needed.
 
-Only the native Windows Cryptographic API (CNG) is supported, see
-**Using Sequoia (Cryptography)** section above.
+[Visual Studio Build Tools]: https://visualstudio.microsoft.com/downloads?q=build+tools
 
 When building, make sure to disable default features (to disable
 Nettle) and enable the CNG via `crypto-cng` Cargo feature:
@@ -326,8 +320,31 @@ Nettle) and enable the CNG via `crypto-cng` Cargo feature:
 $ cargo build --no-default-features --features crypto-cng,compression # Only change crypto backend
 ```
 
-[`capnp`]: https://capnproto.org/install.html
+#### Nettle
+
+It is also possible to use Sequoia's default backend (Nettle) on
+Windows through [MSYS2][].
+
+[MSYS2]: https://www.msys2.org
+
+You can install the needed libraries with the following command:
+
+```shell
+$ pacman -S mingw-w64-x86_64-{bzip2,clang,gcc,pkg-config,nettle}
+```
+
+#### Other
+
+MSYS2 can also be used to build Sequoia with the Windows-native CNG
+backend.  The list of packages is the same as for Nettle with the
+exception of `mingw-w64-x86_64-nettle` which is not needed.  Build
+command is the same as for the CNG backend.
+
+Sequoia PGP can also be built for 32-bit Windows.  See
+`.gitlab-ci.yml` for detailed example.
 
+Additionally, the experimental Rust backend can also be used on
+Windows. See the `sequoia-openpgp` crate's documentation for details.
 
 Getting help
 ============