build: fix glslang compilation on Windows with MSVC (#11801)

> [!WARNING]
> Review/approve this AFTER #11798 and #11800 (this PR stacks on top of
rhem... ergo, it includes their commits)
> Don't cheat! Start from the oldest one! 😄 I know these are almost
one-liners but I am doing this mostly for documentation and karma
points.

## Summary
- Conditionally skip `linkLibCpp()` on MSVC since Zig's bundled libc++
headers conflict with MSVC's own C++ runtime
- Add `-std=c++17` flag for C++17 features (std::variant,
std::filesystem, inline variables) that glslang requires

## Context
The exact same `linkLibCpp` fix was applied to `simdutf` and `highway`
in commits 3d581eb92 and b4c529a82 but glslang was missed. Without this
fix, glslang fails with 297 compilation errors on MSVC.

Thanks Claude for the forensic digging. A carpenter should always be
thankful for his tools. Even if they are borrowed, maybe even more so.

## Stack
Stacked on 011-windows/fix-oniguruma-msvc.

## Discussion points

**`-std=c++17` scope:** Currently added unconditionally for all targets.
Tested on all three platforms with no regressions, but since this is
specifically fixing a Windows/MSVC issue, it could be gated behind
`target.result.abi == .msvc`. Donno. The reason it works unconditionally
is that Zig's bundled clang already defaults to C++17 on non-MSVC
targets, so the flag is a no-op there. Open to either approach.

**Other packages with bare `linkLibCpp()`:** The same `linkLibCpp` guard
has been applied to `simdutf`, `highway`, `utfcpp`, and now `glslang`.
However, `spirv-cross`, `dcimgui`, `harfbuzz`, and `breakpad` still have
unconditional `linkLibCpp()` calls. These may need the same treatment
when they become buildable on MSVC (some are currently blocked by other
issues like freetype's `unistd.h`). Worth tracking as a follow-up?

## Test plan

### test-lib-vt

| | Windows | Linux | Mac |
|---|---|---|---|
| **BEFORE** | 3791/3839 passed, 48 skipped | 3791/3839 passed, 48
skipped | 3807/3839 passed, 32 skipped |
| **AFTER** | 3791/3839 passed, 48 skipped | 3791/3839 passed, 48
skipped | 3807/3839 passed, 32 skipped |
| **Delta** | no change | no change | no change |

### all tests (`zig build test` / `zig build -Dapp-runtime=none test` on
Windows)

| | Windows | Linux | Mac |
|---|---|---|---|
| **BEFORE** | FAIL — 38/51 build steps, 5 failed | 2655/2678 passed, 23
skipped (86/86 steps) | 2655/2662 passed, 7 skipped (160/160 steps) |
| **AFTER** | FAIL — 39/51 build steps, 4 failed | 2655/2678 passed, 23
skipped (86/86 steps) | 2655/2662 passed, 7 skipped (160/160 steps) |
| **Delta** | +1 build step (glslang unblocked) | no change | no change
|

- Zero regressions on any platform
- Windows improved: glslang now compiles (38 -> 39 steps, 5 -> 4
failures)
- Remaining 4 Windows failures (`helpgen`, `framegen`, `freetype`,
`translate-c`) are addressed by other PRs in the stack

## What I Learnt

- **MSVC's clang doesn't default to C++17.** Zig's bundled clang uses
C++17 by default on Linux/Mac, but when targeting MSVC, the C++ standard
needs to be specified explicitly. Without `-std=c++17`, features like
`std::variant`, `std::filesystem`, and `inline` variables are gated
behind `_HAS_CXX17` and won't compile.
- **`linkLibCpp` conflicts with MSVC headers.** Zig's `linkLibCpp`
passes `-nostdinc++` and adds its own libc++/libc++abi headers, which
collide with the C++ headers already provided by the MSVC SDK through
`linkLibC`. On MSVC, you don't need `linkLibCpp` at all since the SDK
includes both C and C++ headers. I think yesterday we dealt with
something similar. Windows is fun. 🫠 Unironically and chronically.
- **Grep wider.** The `linkLibCpp` guard was already applied to simdutf,
highway, and utfcpp but missed glslang. When a fix follows a repeated
pattern across packages, search the whole codebase before declaring it
complete.

pkg/glslang/build.zig

@@ -51,7 +51,14 @@ fn buildGlslang(
         .linkage = .static,
     });
     lib.linkLibC();
-    lib.linkLibCpp();
+    // On MSVC, we must not use linkLibCpp because Zig unconditionally
+    // passes -nostdinc++ and then adds its bundled libc++/libc++abi
+    // include paths, which conflict with MSVC's own C++ runtime headers.
+    // The MSVC SDK include directories (added via linkLibC) contain
+    // both C and C++ headers, so linkLibCpp is not needed.
+    if (target.result.abi != .msvc) {
+        lib.linkLibCpp();
+    }
     if (upstream_) |upstream| lib.addIncludePath(upstream.path(""));
     lib.addIncludePath(b.path("override"));
     if (target.result.os.tag.isDarwin()) {
@@ -65,6 +72,10 @@ fn buildGlslang(
         "-fno-sanitize=undefined",
         "-fno-sanitize-trap=undefined",
     });
+    // MSVC requires explicit std specification otherwise C++17 features
+    // like std::variant, std::filesystem, and inline variables are
+    // guarded behind _HAS_CXX17.
+    try flags.append(b.allocator, "-std=c++17");
 
     if (target.result.os.tag == .freebsd or target.result.abi == .musl) {
         try flags.append(b.allocator, "-fPIC");