Use static only when debugging PixelThresholdClusterizer

[Dr15Jones]

Helgrind reported a race condition with a function static. The
value of the static is only used during a compile time conditional
debugging. Therefore the static was moved to also be within the
same conditional build flag.
Switched the static to std::atomic so when conditionally compiled
the code will be thread safe.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_2_X.

Use static only when debugging PixelThresholdClusterizer

It involves the following packages:

RecoLocalTracker/SiPixelClusterizer

@nclopezo, @cmsbuild, @Degano, @StoyanStoynev, @slava77 can you please review it and eventually sign? Thanks.
@GiacomoSguazzoni, @rovere, @VinInn, @dkotlins, @gpetruc, @cerati, @threus, @venturia this is something you requested to watch as well.
You can sign-off by replying to this message having '+1' in the first line of your reply.
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[cmsbuild]

+1
Tested at: 55c92bb
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-5065/712/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-5065/712/summary.html

[slava77]

+1

for #5065 55c92bb

code change ok
no diffs as expected

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_2_X IBs unless changes (tests are also fine).