[libc++] Introduce LIBCXX_LIBRARY_VERSION

[ldionne]

Summary

This allows controlling the current_version linker property on Apple
platforms.

libcxx/CMakeLists.txt

cmake_dependent_option(LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY
         "LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY;LIBCXX_INSTALL_LIBRARY" OFF)
 
 option(LIBCXX_ABI_UNSTABLE "Use the unstable ABI of libc++. This is equivalent to specifying LIBCXX_ABI_VERSION=n, where n is the not-yet-stable version." OFF)
 if (LIBCXX_ABI_UNSTABLE)
   set(abi_version "2")
 else()
   set(abi_version "1")
 endif()
-set(LIBCXX_ABI_VERSION "${abi_version}" CACHE STRING "ABI version of libc++. Can be either 1 or 2, where 2 is currently the unstable ABI. Defaults to 1 unless LIBCXX_ABI_UNSTABLE is specified, in which case this is 2.")
+set(LIBCXX_ABI_VERSION "${abi_version}" CACHE STRING
+  "ABI version of libc++. Can be either 1 or 2, where 2 is currently the unstable ABI.
+   Defaults to 1 unless LIBCXX_ABI_UNSTABLE is specified, in which case this is 2.")
+set(LIBCXX_LIBRARY_VERSION "${LIBCXX_ABI_VERSION}.0" CACHE STRING
+  "Version of libc++. This will be reflected in the name of the shared library produced.
+   For example, -DLIBCXX_LIBRARY_VERSION=x.y will result in the library being named
+   libc++.x.y.dylib, along with the usual symlinks pointing to that. On Apple platforms,
+   this also controls the linker's 'current_version' property.")
 set(LIBCXX_ABI_NAMESPACE "__${LIBCXX_ABI_VERSION}" CACHE STRING "The inline ABI namespace used by libc++. It defaults to __n where `n` is the current ABI version.")
 if (NOT LIBCXX_ABI_NAMESPACE MATCHES "__.*")
   message(FATAL_ERROR "LIBCXX_ABI_NAMESPACE must be a reserved identifier.")
 endif()
 option(LIBCXX_ABI_FORCE_ITANIUM "Ignore auto-detection and force use of the Itanium ABI.")
 option(LIBCXX_ABI_FORCE_MICROSOFT "Ignore auto-detection and force use of the Microsoft ABI.")
 
 set(LIBCXX_TYPEINFO_COMPARISON_IMPLEMENTATION "default" CACHE STRING

libcxx/src/CMakeLists.txt

if (LIBCXX_ENABLE_SHARED)
   target_include_directories(cxx_shared PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
   target_link_libraries(cxx_shared PUBLIC cxx-headers
                                    PRIVATE ${LIBCXX_LIBRARIES})
   set_target_properties(cxx_shared
     PROPERTIES
       COMPILE_FLAGS "${LIBCXX_COMPILE_FLAGS}"
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
-      VERSION       "${LIBCXX_ABI_VERSION}.0"
+      VERSION       "${LIBCXX_LIBRARY_VERSION}"
       SOVERSION     "${LIBCXX_ABI_VERSION}"
       DEFINE_SYMBOL ""
   )
   cxx_add_common_build_flags(cxx_shared)
   cxx_set_common_defines(cxx_shared)
 
   # Link against LLVM libunwind
   # Note that we do need to link against libunwind directly to ensure that the correct