ABI versioning macros for libc++.

C++ macros and CMake options that specify the default ABI version of
the library, and can be overridden to pick up new ABI-changing
features.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@250254 91177308-0d34-0410-b5e6-96231b3b80d8

docs/BuildingLibcxx.rst

@@ -193,6 +193,25 @@ libc++ Feature options
 
   Build libc++ with run time type information.
 
+
+libc++ Feature options
+----------------------
+
+The following options allow building libc++ for a different ABI version.
+
+.. option:: LIBCXX_ABI_VERSION:STRING
+
+  **Default**: ``1``
+
+  Defines the target ABI version of libc++.
+
+.. option:: LIBCXX_ABI_UNSTABLE:BOOL
+
+  **Default**: ``OFF``
+
+  Build the "unstable" ABI version of libc++. Includes all ABI changing features
+  on top of the current stable version.
+
 .. _LLVM-specific variables:
 
 LLVM-specific options