[PATCH] [libcxx] Correct ifdef for glibc specific feature. Patch by David Turner <digit at android.com>

[Dan Albert]

For Android, this isn't the whole truth. I actually have a patch waiting for review that covers this: http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20140623/108571.html

http://reviews.llvm.org/D4339