diff --git a/AK/Demangle.h b/AK/Demangle.h
index b377556e4b..42dc1cdae4 100644
--- a/AK/Demangle.h
+++ b/AK/Demangle.h
@@ -29,7 +29,7 @@
 #include <AK/String.h>
 #include <AK/StringView.h>
 
-#ifndef __serenity__
+#ifndef BUILDING_SERENITY_TOOLCHAIN
 #    include <cxxabi.h>
 #endif
 
@@ -37,10 +37,9 @@ namespace AK {
 
 inline String demangle(const StringView& name)
 {
-#ifdef __serenity__
+#ifdef BUILDING_SERENITY_TOOLCHAIN
     return name;
 #else
-    // FIXME: Implement __cxa_demangle in serenity
     int status = 0;
     auto* demangled_name = abi::__cxa_demangle(name.to_string().characters(), nullptr, nullptr, &status);
     auto string = String(status == 0 ? demangled_name : name);
diff --git a/Toolchain/BuildIt.sh b/Toolchain/BuildIt.sh
index ab499897c4..d55dfd58ae 100755
--- a/Toolchain/BuildIt.sh
+++ b/Toolchain/BuildIt.sh
@@ -228,7 +228,7 @@ pushd "$DIR/Build/"
         echo "XXX serenity libc and libm"
         mkdir -p "$BUILD"
         pushd "$BUILD"
-            cmake ..
+            CXXFLAGS="-DBUILDING_SERENITY_TOOLCHAIN" cmake ..
             "$MAKE" LibC
             install -D Libraries/LibC/libc.a Libraries/LibM/libm.a Root/usr/lib/
             SRC_ROOT=$(realpath "$DIR"/..)