diff --git a/CMakeLists.txt b/CMakeLists.txt
index e75ab2d21..e70f869a7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -321,6 +321,9 @@ if (LIBCXX_NEEDS_SITE_CONFIG)
     include/__config_site.in
     ${LIBCXX_BINARY_DIR}/__config_site
     @ONLY)
+  # Provide the config definitions by included the generated __config_site
+  # file at compile time.
+  add_compile_flags("-include ${LIBCXX_BINARY_DIR}/__config_site")
 endif()
 
 #===============================================================================
diff --git a/cmake/Modules/HandleLibcxxFlags.cmake b/cmake/Modules/HandleLibcxxFlags.cmake
index 320650513..bb886fa8a 100644
--- a/cmake/Modules/HandleLibcxxFlags.cmake
+++ b/cmake/Modules/HandleLibcxxFlags.cmake
@@ -49,10 +49,12 @@ macro(define_if_not condition def)
   endif()
 endmacro()
 
+# Add a macro definition to the __config_site file if the specified condition
+# is 'true'. Note that '-D${def}' is not added. Instead it is expected that
+# the build include the '__config_site' header.
 macro(config_define_if condition def)
   if (${condition})
     set(${def} ON)
-    add_definitions(-D${def})
     set(LIBCXX_NEEDS_SITE_CONFIG ON)
   endif()
 endmacro()
@@ -60,14 +62,12 @@ endmacro()
 macro(config_define_if_not condition def)
   if (NOT ${condition})
     set(${def} ON)
-    add_definitions(-D${def})
     set(LIBCXX_NEEDS_SITE_CONFIG ON)
   endif()
 endmacro()
 
 macro(config_define value def)
   set(${def} ${value})
-  add_definitions(-D${def}=${value})
   set(LIBCXX_NEEDS_SITE_CONFIG ON)
 endmacro()
 
diff --git a/docs/Abi.rst b/docs/DesignDocs/ABIVersioning.rst
similarity index 100%
rename from docs/Abi.rst
rename to docs/DesignDocs/ABIVersioning.rst
diff --git a/docs/index.rst b/docs/index.rst
index f34979f6c..78876116c 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -128,6 +128,7 @@ Design Documents
    :maxdepth: 1
 
    DesignDocs/CapturingConfigInfo
+   DesignDocs/ABIVersioning
 
 
 * `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
diff --git a/test/libcxx/test/config.py b/test/libcxx/test/config.py
index 3cc1d3b80..fd9b47628 100644
--- a/test/libcxx/test/config.py
+++ b/test/libcxx/test/config.py
@@ -444,7 +444,9 @@ class Configuration(object):
     def configure_compile_flags_abi_version(self):
         abi_version = self.get_lit_conf('abi_version', '').strip()
         abi_unstable = self.get_lit_bool('abi_unstable')
-        if abi_version:
+        # Only add the ABI version when it is non-default.
+        # FIXME(EricWF): Get the ABI version from the "__config_site".
+        if abi_version and abi_version != '1':
           self.cxx.compile_flags += ['-D_LIBCPP_ABI_VERSION=' + abi_version]
         if abi_unstable:
           self.config.available_features.add('libcpp-abi-unstable')