Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -607,6 +607,18 @@
 config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 
+if (LIBCXX_ABI_DEFINES)
+  set(abi_defines)
+  foreach (abi_define ${LIBCXX_ABI_DEFINES})
+    if (NOT abi_define MATCHES "^_LIBCPP_ABI_")
+      message(SEND_ERROR "Invalid ABI macro ${abi_define} in LIBCXX_ABI_DEFINES")
+    endif()
+    list(APPEND abi_defines "#define ${abi_define}")
+  endforeach()
+  string(REPLACE ";" "\n" abi_defines "${abi_defines}")
+  config_define(${abi_defines} _LIBCPP_ABI_DEFINES)
+endif()
+
 # By default libc++ on Windows expects to use a shared library, which requires
 # the headers to use DLL import/export semantics. However when building a
 # static library only we modify the headers to disable DLL import/export.
Index: include/__config_site.in
===================================================================
--- include/__config_site.in
+++ include/__config_site.in
@@ -24,4 +24,6 @@
 #cmakedefine _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL
 #cmakedefine _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS
 
+@_LIBCPP_ABI_DEFINES@
+
 #endif // _LIBCPP_CONFIG_SITE
Index: utils/libcxx/test/config.py
===================================================================
--- utils/libcxx/test/config.py
+++ utils/libcxx/test/config.py
@@ -658,7 +658,7 @@
                 self.config.available_features.add('libcpp-abi-version-v%s'
                     % feature_macros[m])
                 continue
-            assert m.startswith('_LIBCPP_HAS_') or m == '_LIBCPP_ABI_UNSTABLE'
+            assert m.startswith('_LIBCPP_HAS_') or m.startswith('_LIBCPP_ABI_')
             m = m.lower()[1:].replace('_', '-')
             self.config.available_features.add(m)
         return feature_macros