fix: add -nostdinc++ -stdlib=libc++ for Clang --no-default-config

When bypassing clang++.cfg, the cfg's -nostdinc++ and -stdlib=libc++
flags are also stripped. Without -nostdinc++, Clang may find host
libstdc++ headers before the payload's libc++ headers. Without
-stdlib=libc++, Clang defaults to libstdc++ runtime.

Also remove the /usr/include fallback for linux-headers — mixing
host headers with xpkg glibc causes bits/wordsize.h conflicts.

Author: Sunrisepeak

src/build/flags.cppm

@@ -111,7 +111,7 @@ CompileFlags compute_flags(const BuildPlan& plan) {
         // Clang with cfg: bypass cfg and provide all paths explicitly.
         auto llvmRoot = plan.toolchain.binaryPath.parent_path().parent_path();
         auto libcxxInclude = llvmRoot / "include" / "c++" / "v1";
-        sysroot_flag = " --no-default-config";
+        sysroot_flag = " --no-default-config -nostdinc++";
         // libc++ headers
         sysroot_flag += " -stdlib=libc++";
         sysroot_flag += " -isystem" + escape_path(libcxxInclude);

src/toolchain/probe.cppm

@@ -342,12 +342,6 @@ probe_payload_paths(const std::filesystem::path& compilerBin) {
             pp.linuxInclude = linuxInclude;
     }
 
-    // Fallback: host /usr/include has linux kernel headers on most systems.
-    if (pp.linuxInclude.empty()) {
-        if (std::filesystem::exists("/usr/include/linux/limits.h"))
-            pp.linuxInclude = "/usr/include";
-    }
-
     return pp;
 }
 

src/toolchain/stdmod.cppm

@@ -108,7 +108,7 @@ std::expected<StdModule, StdModError> ensure_built(
         if (std::filesystem::exists(cfgPath)) {
             auto llvmRoot = tc.binaryPath.parent_path().parent_path();
             auto libcxxInclude = llvmRoot / "include" / "c++" / "v1";
-            sysroot_flag = " --no-default-config";
+            sysroot_flag = " --no-default-config -nostdinc++ -stdlib=libc++";
             sysroot_flag += std::format(" -isystem'{}'", libcxxInclude.string());
             if (!tc.targetTriple.empty()) {
                 auto targetInclude = llvmRoot / "include"