build: add -std=c++17 for SIMD C++ files on MSVC

The SIMD C++ files use C++17 features (std::optional, std::size).
With Zig's bundled libc++ these are available implicitly, but MSVC
headers guard C++17 features behind the standard version
(_HAS_CXX17). Without an explicit -std=c++17 flag, clang defaults
to a lower standard and the MSVC <optional> header does not define
std::optional.

src/build/SharedDeps.zig

@@ -790,12 +790,27 @@ pub fn addSimd(
         const HWY_AVX3_DL: c_int = 1 << 7;
         const HWY_AVX3: c_int = 1 << 8;
 
+        var flags: std.ArrayListUnmanaged([]const u8) = .empty;
+
         // Zig 0.13 bug: https://github.com/ziglang/zig/issues/20414
         // To workaround this we just disable AVX512 support completely.
         // The performance difference between AVX2 and AVX512 is not
         // significant for our use case and AVX512 is very rare on consumer
         // hardware anyways.
         const HWY_DISABLED_TARGETS: c_int = HWY_AVX10_2 | HWY_AVX3_SPR | HWY_AVX3_ZEN4 | HWY_AVX3_DL | HWY_AVX3;
+        if (target.result.cpu.arch == .x86_64) try flags.append(
+            b.allocator,
+            b.fmt("-DHWY_DISABLED_TARGETS={}", .{HWY_DISABLED_TARGETS}),
+        );
+
+        // MSVC requires explicit std specification otherwise these
+        // are guarded, at least on Windows 2025. Doing it unconditionally
+        // doesn't cause any issues on other platforms and ensures we get
+        // C++17 support on MSVC.
+        try flags.append(
+            b.allocator,
+            "-std=c++17",
+        );
 
         m.addCSourceFiles(.{
             .files = &.{
@@ -804,9 +819,7 @@ pub fn addSimd(
                 "src/simd/index_of.cpp",
                 "src/simd/vt.cpp",
             },
-            .flags = if (target.result.cpu.arch == .x86_64) &.{
+            .flags = flags.items,
-                b.fmt("-DHWY_DISABLED_TARGETS={}", .{HWY_DISABLED_TARGETS}),
-            } else &.{},
         });
     }
 }