MFC r320240:

MFC r320240:

  Use __ISO_C_VISIBLE, as opposed to testing __STDC_VERSION__.

  FreeBSD's C library uses __STDC_VERSION__ to determine whether the
  compiler provides language features specific to a certain version of the
  C standard. __ISO_C_VISIBLE is used to specify which library features
  need to be exposed.

  max_align_t currently uses __STDC_VERSION__, even though it should be
  using __ISO_C_VISIBLE to remain consistent with the rest of the headers
  in include/.