gmacros: Only use deprecated attributes on enumerators with GCC ≥ 6.5
The reference I found online which said they were supported by GCC ≥ 5.2 was obviously lying.
The GCC 6.5.0 documentation says they’re supported: https://gcc.gnu.org/onlinedocs/gcc-6.5.0/gcc/Enumerator-Attributes.html.
Signed-off-by: Philip Withnall withnall@endlessm.com