[libc++abi] NFC: Add link to review in workaround comment
To avoid wasting the valuable time of contributors, add a link to a
blocked review to document additional issues with the removal of some
GCC 4.9 workaround.
diff --git a/libcxxabi/CMakeLists.txt b/libcxxabi/CMakeLists.txt
index a58789f..a269806 100644
--- a/libcxxabi/CMakeLists.txt
+++ b/libcxxabi/CMakeLists.txt
@@ -69,6 +69,7 @@
# FIXME: This option should default to off. Unfortunatly GCC 4.9 fails to link
# programs to due undefined references to new/delete in libc++abi. Once this
# has been fixed or worked around the default value should be changed.
+# See https://reviews.llvm.org/D68269 for more details.
option(LIBCXXABI_ENABLE_NEW_DELETE_DEFINITIONS
"Build libc++abi with definitions for operator new/delete. Normally libc++
provides these definitions" ON)
