Add DirectX SDK (D3DX9) download support

download --with-dxsdk fetches the DirectX SDK (June 2010) - the last
standalone release of D3DX9/10/11, XInput and XAudio2, dropped from
the Windows SDK entirely once D3DX was deprecated, but still needed
by plenty of legacy code. Like the WDK, it isn't part of the VS
installer manifest, so this is its own self-contained fetch+unpack
path: the installer is a self-extracting CAB, unpacked directly via
cabextract (already a prerequisite for the WinSDK .msi payloads) with
its -F filter restricting extraction to just Include/ and Lib/ -
about 21MB out of the installer's 1.2GB uncompressed payload.

Verified against the real installer: real d3dx9.h and d3dx9.lib
(x86 and x64) extracted correctly and linked into an actual legacy
game client build.

README.md

@@ -8,8 +8,9 @@ vintner cross-compiles with the real MSVC toolchain on Linux, using Wine.
 One Go binary drops in as `cl`, `link`, `lib`, `rc`, `midl`, `mc`, `mt`,
 `dumpbin`, `msbuild`, `nmake`, `ml`, `ml64`, `armasm`, `armasm64`, plus
 `cmd`/`findstr` shims, so once installed you invoke the real Microsoft
-tools exactly like on Windows. It handles full MSBuild projects, and with
-`--with-wdk`, real KMDF/UMDF Windows drivers.
+tools exactly like on Windows. It handles full MSBuild projects, with
+`--with-wdk` real KMDF/UMDF Windows drivers, and with `--with-dxsdk` the
+real D3DX9 headers/libs.
 
 Inspired by [mstorsjo/msvc-wine](https://github.com/mstorsjo/msvc-wine)'s
 approach: download the real MSVC/WinSDK, wrap the compiler under Wine.
@@ -21,6 +22,7 @@ approach: download the real MSVC/WinSDK, wrap the compiler under Wine.
 - [Quick start](#quick-start)
 - [Commands](#commands)
 - [Building drivers (WDK)](#building-drivers-wdk)
+- [Building against D3DX9 (DirectX SDK)](#building-against-d3dx9-directx-sdk)
 - [Language](#language)
 - [Shell completion](#shell-completion)
 - [Using clang-cl/lld-link instead of Wine](#using-clang-cllld-link-instead-of-wine)
@@ -112,12 +114,13 @@ vintner completion bash|zsh                                       print a shell
 
 `download`'s main options: `--msvc-version`, `--sdk-version`,
 `--architecture` (repeatable: `x86`/`x64`/`arm`/`arm64`/`host`),
-`--host-arch`, `--only-host`, `--with-wdk` (see below), `--ignore`
-(repeatable), `--only-download`, `--only-unpack`, `--keep-unpack`,
-`--skip-patch`, `--cache`, `--language`, `--include-optional`,
-`--skip-recommended`, `--major`, `--preview`, `--manifest`,
-`--list-workloads`, `--list-components`, `--print-deps-tree`. Run
-`vintner download -h` for the full list with descriptions.
+`--host-arch`, `--only-host`, `--with-wdk` (see below), `--with-dxsdk`
+(see below), `--ignore` (repeatable), `--only-download`, `--only-unpack`,
+`--keep-unpack`, `--skip-patch`, `--cache`, `--language`,
+`--include-optional`, `--skip-recommended`, `--major`, `--preview`,
+`--manifest`, `--list-workloads`, `--list-components`,
+`--print-deps-tree`. Run `vintner download -h` for the full list with
+descriptions.
 
 `--list-workloads`/`--list-components` print every workload/component id
 and its human-readable title from the fetched manifest, then exit
@@ -143,6 +146,22 @@ work under Wine. Tested against a real sample driver from
 Only x64 and arm64 targets have a WDK package upstream; there's no x86 or
 arm one.
 
+## Building against D3DX9 (DirectX SDK)
+
+`--with-dxsdk` fetches the DirectX SDK (June 2010) — the last standalone
+release of D3DX9/10/11, XInput and XAudio2, dropped from the Windows SDK
+entirely once D3DX was deprecated. It unpacks the real headers and x86/x64
+import libs (`d3dx9.h`/`d3dx9.lib` included) to `<dest>/DXSDK`.
+
+```bash
+vintner download --accept-license --with-dxsdk
+```
+
+Point your project's `IncludePath`/`LibraryPath` at
+`<dest>/DXSDK/Include` and `<dest>/DXSDK/Lib/x86` or `<dest>/DXSDK/Lib/x64`.
+Requires `cabextract` on `PATH` (the installer is a self-extracting CAB
+archive).
+
 ## Language
 
 CLI text (usage, progress lines, prompts) defaults to English. Set