Use __config_site when building libc++. Also cleanup ABI versioning doc

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@250261 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e75ab2d..e70f869 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -321,6 +321,9 @@
     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()
 
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