More work on c/cpp generators.
The new -check true mode should avoid spurious data races
Consider for instance MP+ponarel+poacqna that is racy
with -check false and is not with -check true,
as the second load (value) is performed only when the first load
(flag) reads the value 1.
Resolved conflict by hand-merging

Merge branch 'master' of github.com:herd/herdtools

Conflicts:
	litmus/top.ml