diff --git a/internal/rosa/go.go b/internal/rosa/go.go
index 456cc9c..d1dca75 100644
--- a/internal/rosa/go.go
+++ b/internal/rosa/go.go
@@ -31,7 +31,7 @@ rm \
 	syscall/creds_test.go \
 	net/multicast_test.go
 
-CC="${CC} ${LDFLAGS}" ./all.bash
+CC="cc -Qunused-arguments ${LDFLAGS}" ./all.bash
 `, pkg.Path(AbsUsrSrc.Append("go1.4-bootstrap"), false, pkg.NewHTTPGetTar(
 		nil, "https://dl.google.com/go/go1.4-bootstrap-20171003.tar.gz",
 		mustDecode(checksum),
@@ -48,6 +48,7 @@ func (t Toolchain) newGo(
 	return t.New("go"+version, []pkg.Artifact{
 		boot,
 	}, nil, slices.Concat([]string{
+		"CC=cc",
 		"GOCACHE=/tmp/gocache",
 		"GOROOT_BOOTSTRAP=/system/go",
 		"CGO_" + ldflags(false) + " -O2 -g",
diff --git a/internal/rosa/llvm.go b/internal/rosa/llvm.go
index d10fb56..70752a5 100644
--- a/internal/rosa/llvm.go
+++ b/internal/rosa/llvm.go
@@ -310,13 +310,19 @@ ln -s \
 		},
 	})
 
+	var clangExtraEnv []string
+	if t == toolchainStage3 {
+		clangExtraEnv = []string{
+			"CFLAGS=" + earlyCFLAGS,
+			"CXXFLAGS=" + earlyCXXFLAGS(),
+		}
+	}
+
 	clang = t.newLLVM("clang", &llvmAttr{
 		flags: llvmProjectClang | llvmProjectLld,
-		env: []string{
-			"CFLAGS=" + cflags,
-			"CXXFLAGS=" + cxxflags(),
+		env: slices.Concat(clangExtraEnv, []string{
 			ldflags(false),
-		},
+		}),
 		cmake: slices.Concat([][2]string{
 			{"LLVM_TARGETS_TO_BUILD", target},
 			{"CMAKE_CROSSCOMPILING", "OFF"},
@@ -329,6 +335,9 @@ ln -s \
 			t.NewGit(),
 		},
 		script: `
+ln -s clang /work/system/bin/cc
+ln -s clang++ /work/system/bin/c++
+
 ninja check-all
 `,
 
@@ -353,6 +362,33 @@ index 50fdce630255..4b4465a75617 100644
  /// Verify timestamps that gets embedded in the module
  #include <c-header.h>
 
+`},
+
+			{"path-system-include", `diff --git a/clang/lib/Driver/ToolChains/Linux.cpp b/clang/lib/Driver/ToolChains/Linux.cpp
+index cdbf21fb9026..dd052858700d 100644
+--- a/clang/lib/Driver/ToolChains/Linux.cpp
++++ b/clang/lib/Driver/ToolChains/Linux.cpp
+@@ -773,6 +773,12 @@ void Linux::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
+     addExternCSystemInclude(
+         DriverArgs, CC1Args,
+         concat(SysRoot, "/usr/include", MultiarchIncludeDir));
++  if (!MultiarchIncludeDir.empty() &&
++      D.getVFS().exists(concat(SysRoot, "/system/include", MultiarchIncludeDir)))
++    addExternCSystemInclude(
++        DriverArgs, CC1Args,
++        concat(SysRoot, "/system/include", MultiarchIncludeDir));
++
+
+   if (getTriple().getOS() == llvm::Triple::RTEMS)
+     return;
+@@ -783,6 +789,7 @@ void Linux::AddClangSystemIncludeArgs(const ArgList &DriverArgs,
+   addExternCSystemInclude(DriverArgs, CC1Args, concat(SysRoot, "/include"));
+
+   addExternCSystemInclude(DriverArgs, CC1Args, concat(SysRoot, "/usr/include"));
++  addExternCSystemInclude(DriverArgs, CC1Args, concat(SysRoot, "/system/include"));
+
+   if (!DriverArgs.hasArg(options::OPT_nobuiltininc) && getTriple().isMusl())
+     addSystemInclude(DriverArgs, CC1Args, ResourceDirInclude);
 `},
 		},
 	})
diff --git a/internal/rosa/perl.go b/internal/rosa/perl.go
index 169a64d..be36bdf 100644
--- a/internal/rosa/perl.go
+++ b/internal/rosa/perl.go
@@ -16,7 +16,7 @@ chmod -R +w /usr/src/perl && cd /usr/src/perl
 ./Configure \
 	-des \
 	-Dprefix=/system \
-	-Dcc="${CC}" \
+	-Dcc="clang" \
 	-Dcflags='--std=gnu99' \
 	-Dldflags="${LDFLAGS}" \
 	-Doptimize='-O2 -fno-strict-aliasing' \
diff --git a/internal/rosa/python.go b/internal/rosa/python.go
index 8fbc74f..7c05927 100644
--- a/internal/rosa/python.go
+++ b/internal/rosa/python.go
@@ -46,7 +46,8 @@ chmod -R +w /usr/src/python/
 export HOME="$(mktemp -d)"
 cd "$(mktemp -d)"
 /usr/src/python/configure \
-	--prefix=/system
+	--prefix=/system \
+	--build="${ROSA_TRIPLE}"
 make "-j$(nproc)"
 make test
 make DESTDIR=/work install
diff --git a/internal/rosa/rosa.go b/internal/rosa/rosa.go
index c679940..795394c 100644
--- a/internal/rosa/rosa.go
+++ b/internal/rosa/rosa.go
@@ -63,10 +63,6 @@ func triplet() string {
 const (
 	// EnvTriplet holds the return value of triplet.
 	EnvTriplet = "ROSA_TRIPLE"
-	// EnvRefCFLAGS holds toolchain-specific reference CFLAGS.
-	EnvRefCFLAGS = "ROSA_CFLAGS"
-	// EnvRefCXXFLAGS holds toolchain-specific reference CXXFLAGS.
-	EnvRefCXXFLAGS = "ROSA_CXXFLAGS"
 )
 
 // ldflags returns LDFLAGS corresponding to triplet.
@@ -85,13 +81,13 @@ func ldflags(static bool) string {
 	return s
 }
 
-// cflags is reference CFLAGS for the Rosa OS toolchain.
-const cflags = "-Qunused-arguments " +
+// earlyCFLAGS is reference CFLAGS for the stage3 toolchain.
+const earlyCFLAGS = "-Qunused-arguments " +
 	"-isystem/system/include"
 
-// cxxflags returns reference CXXFLAGS for the Rosa OS toolchain corresponding
-// to [runtime.GOARCH].
-func cxxflags() string {
+// earlyCXXFLAGS returns reference CXXFLAGS for the stage3 toolchain
+// corresponding to [runtime.GOARCH].
+func earlyCXXFLAGS() string {
 	return "--start-no-unused-arguments " +
 		"-stdlib=libc++ " +
 		"--end-no-unused-arguments " +
@@ -203,9 +199,6 @@ ln -vs ../usr/bin /work/bin
 		env = fixupEnviron(env, []string{
 			EnvTriplet + "=" + triplet(),
 			lcMessages,
-
-			EnvRefCFLAGS + "=" + cflags,
-			EnvRefCXXFLAGS + "=" + cxxflags(),
 			ldflags(true),
 		}, "/system/bin",
 			"/usr/bin",
@@ -227,11 +220,6 @@ ln -vs ../usr/bin /work/bin
 			EnvTriplet + "=" + triplet(),
 			lcMessages,
 
-			// autotools projects act up with CFLAGS
-			"CC=clang " + cflags,
-			EnvRefCFLAGS + "=" + cflags,
-			"CXX=clang++ " + cxxflags(),
-			EnvRefCXXFLAGS + "=" + cxxflags(),
 			ldflags(false),
 
 			"AR=ar",
diff --git a/internal/rosa/zlib.go b/internal/rosa/zlib.go
index 203514a..4655f19 100644
--- a/internal/rosa/zlib.go
+++ b/internal/rosa/zlib.go
@@ -12,7 +12,7 @@ func (t Toolchain) NewZlib() pkg.Artifact {
 		t.NewMake(),
 	}, nil, nil, `
 cd "$(mktemp -d)"
-CFLAGS="${CFLAGS} -fPIC" /usr/src/zlib/configure \
+CC="clang -fPIC" /usr/src/zlib/configure \
 	--prefix /system
 make "-j$(nproc)" test
 make DESTDIR=/work install