libghostty: use no_libc mode for simd

pkg/simdutf/build.zig

@@ -85,6 +85,7 @@ pub fn build(b: *std.Build) !void {
                 .target = target,
                 .optimize = optimize,
                 .link_libc = false,
+                .pic = true,
             }),
         });
         lib.addObject(no_libc_obj);

src/build/GhosttyZig.zig

@@ -119,11 +119,11 @@ fn initVt(
         .target = cfg.target,
         .optimize = cfg.optimize,
 
-        // SIMD requires libc. Vendored C++ dependencies are built with
-        // no-libcxx mode (HWY_NO_LIBCXX / SIMDUTF_NO_LIBCXX) so we
-        // don't need libcpp. System-provided simdutf headers still
-        // use C++ stdlib headers, so we need libcpp in that case.
-        .link_libc = if (cfg.simd) true else null,
+        // Vendored C++ dependencies are built with no-libcxx and
+        // no-libc modes so we don't need libc or libcpp. System-provided
+        // simdutf requires both libc and libcpp at runtime.
+        .link_libc = if (cfg.simd and
+            b.systemIntegrationOption("simdutf", .{})) true else null,
         .link_libcpp = if (cfg.simd and
             b.systemIntegrationOption("simdutf", .{}) and
             cfg.target.result.abi != .msvc) true else null,
@@ -138,9 +138,12 @@ fn initVt(
     deps.addUucode(b, vt, cfg.target, cfg.optimize);
 
     // If SIMD is enabled, add all our SIMD dependencies.
-    if (cfg.simd) {
-        try SharedDeps.addSimd(b, vt, simd_libs);
-    }
+    if (cfg.simd) try SharedDeps.addSimd(
+        b,
+        vt,
+        simd_libs,
+        if (cfg.emit_lib_vt) .no_libc else .libc,
+    );
 
     return vt;
 }

src/build/SharedDeps.zig

@@ -381,6 +381,7 @@ pub fn add(
         b,
         step.root_module,
         &static_libs,
+        .libc,
     );
 
     // Wasm we do manually since it is such a different build.
@@ -755,10 +756,13 @@ fn addGtkNg(
 
 /// Add only the dependencies required for `Config.simd` enabled. This also
 /// adds all the simd source files for compilation.
+const SimdLibc = enum { libc, no_libc };
+
 pub fn addSimd(
     b: *std.Build,
     m: *std.Build.Module,
     static_libs: ?*LazyPathList,
+    simd_libc: SimdLibc,
 ) !void {
     const target = m.resolved_target.?;
     const optimize = m.optimize.?;
@@ -772,6 +776,7 @@ pub fn addSimd(
             .target = target,
             .optimize = optimize,
             .no_libcxx = true,
+            .no_libc = simd_libc == .no_libc,
         })) |simdutf_dep| {
             m.linkLibrary(simdutf_dep.artifact("simdutf"));
             if (static_libs) |v| try v.append(