Certain versions of GCC (~4.7) couldn't handle the SFINAE on access
control, but with "= delete" (hidden behind a macro for portability)
this issue is worked around/addressed.
Patch by Agustín Bergé
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211525 91177308-0d34-0410-b5e6-96231b3b80d8