fix: add target-specific libc++ include path for --no-default-config

When bypassing clang++.cfg with --no-default-config, we must provide
both libc++ include paths that the cfg originally supplied:
  -isystem <llvmRoot>/include/c++/v1
  -isystem <llvmRoot>/include/<triple>/c++/v1

The target-specific path contains __config_site which is required by
__config. Without it, std module precompilation fails with
'__config_site' file not found.

Author: Sunrisepeak

src/build/flags.cppm

@@ -102,6 +102,14 @@ CompileFlags compute_flags(const BuildPlan& plan) {
             auto libcxxInclude = llvmRoot / "include" / "c++" / "v1";
             sysroot_flag = " --no-default-config";
             sysroot_flag += " -isystem" + escape_path(libcxxInclude);
+            // Target-specific libc++ headers (e.g. __config_site) live under
+            // include/<triple>/c++/v1/. Add if present.
+            if (!plan.toolchain.targetTriple.empty()) {
+                auto targetInclude = llvmRoot / "include"
+                                     / plan.toolchain.targetTriple / "c++" / "v1";
+                if (std::filesystem::exists(targetInclude))
+                    sysroot_flag += " -isystem" + escape_path(targetInclude);
+            }
             if (!plan.toolchain.sysroot.empty())
                 sysroot_flag += " --sysroot=" + escape_path(plan.toolchain.sysroot);
             else if (auto sdk = mcpp::platform::macos::sdk_path())

src/toolchain/stdmod.cppm

@@ -109,6 +109,14 @@ std::expected<StdModule, StdModError> ensure_built(
             auto libcxxInclude = llvmRoot / "include" / "c++" / "v1";
             sysroot_flag = " --no-default-config";
             sysroot_flag += std::format(" -isystem'{}'", libcxxInclude.string());
+            // Target-specific libc++ headers (e.g. __config_site) live under
+            // include/<triple>/c++/v1/. Add if present.
+            if (!tc.targetTriple.empty()) {
+                auto targetInclude = llvmRoot / "include"
+                                     / tc.targetTriple / "c++" / "v1";
+                if (std::filesystem::exists(targetInclude))
+                    sysroot_flag += std::format(" -isystem'{}'", targetInclude.string());
+            }
             if (!tc.sysroot.empty())
                 sysroot_flag += std::format(" --sysroot='{}'", tc.sysroot.string());
             else if (auto sdk = mcpp::platform::macos::sdk_path())