Add gcc memory barriers after the manner of emacs20's patch-bm, for
the same reason: gcc "knows" that malloc has no side effects and
reorders code around it, only it's wrong. Fixes SIGSEGV during build
seen in some environments.

Bump package revision as a precaution, because I don't understand why
this sometimes doesn't fail and sometimes does with the same gcc
version.

(dholland)