Correct handling of enum attributes with g++

From OpenBSD's commit log:

This was responsible for memory corruption with recent versions
of Mesa where c and c++ code share a header with a packed enum type.

Reference:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39219

Obtained from: OpenBSD (CVS rev. 1.2)
MFC after: 1 week