Remove FIXME for feature test macro
Remove FIXME for feature test macro

The value I'd picked was correct, as per the recently published SG10 paper http://wg21.link/p0096r3

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@276309 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx