Fix some -Wundef issues.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@228266 91177308-0d34-0410-b5e6-96231b3b80d8
Change-Id: Ib53cc3c6d89254376a34d795f6bf34b32f7c1577
(cherry picked from commit 3727be521a9b4c5fcf95c53adc8abe85f7105188)
1 file changed