net/megacmd: fix configure error on i368

[Robert Bagdan <kikadf.01%gmail.com@localhost>]

Hi,
I fixed megacmd configure error on i386: cannot figure out how to use
std::atomic,
with use atomic64.mk. I tested, megacmd builds and works with changes
on i386 as well.

Changes available on pkgsrc-wip/megacmd.

Regards,
kikadf