Correct handling of enum attributes with g++

Correct handling of enum attributes with g++

gcc and g++ can currently have different ideas on the size of a
packed enum type:

enum __attribute__((packed)) foo { a = 0, b};

gcc: 1
g++: 4

enum foo { a = 0, b} __attribute__((packed));

gcc: 1
g++: 1

The first format is actually the preferred one according to the
documentation.
https://gcc.gnu.org/onlinedocs/gcc-4.2.1/gcc/Type-Attributes.html
g++ will accept the first format and silently not actually choose a
smaller size.

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

The problem was reported in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=39219
and fixed in gcc >= 4.3.6 in rev 144284.
This was after the switch from gplv2 but it's a trivial one line change.

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