Fix h2xs enum handling with C++ comments

Fix h2xs enum handling with C++ comments

Contrary to the comment in the code, h2xs mishandles enums that
contain C++ style comments.

An example of a failing header:
enum {
    A = -1, // negative one
            // with more comments
    B = -2, // negative two
    C = -3 // negative two
};

which generates exported constants for 'A' and 'negative'.

The patch is slightly modified from the one by Daniel Burr in
http://bugs.debian.org/320286