diff --git a/src/buildtools/third_party/libc++/__config_site b/src/buildtools/third_party/libc++/__config_site
index 67075bd8b4..83868ae7e2 100644
--- a/src/buildtools/third_party/libc++/__config_site
+++ b/src/buildtools/third_party/libc++/__config_site
@@ -25,7 +25,9 @@
 #define _LIBCPP_HAS_THREADS 1
 #define _LIBCPP_HAS_MONOTONIC_CLOCK 1
 #define _LIBCPP_HAS_TERMINAL 1
+#ifndef _LIBCPP_HAS_MUSL_LIBC
 #define _LIBCPP_HAS_MUSL_LIBC 0
+#endif
 
 #ifdef _WIN32
 #define _LIBCPP_HAS_THREAD_API_PTHREAD 0