Build sherpa with std=c++17

[iarspider]

Try building Sherpa with std=c++17 (same as ROOT, which is a Sherpa dependency). It is currently built with std=c++0x (which is an archaic form of std=c++11)

[iarspider]

please test

[cmsbuild]

A new Pull Request was created by @iarspider for branch IB/CMSSW_12_4_X/master.

@smuzaffar, @iarspider can you please review it and eventually sign? Thanks.
@perrotta, @dpiparo, @qliphy you are the release manager for this.
cms-bot commands are listed here

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-fdfa10/23250/summary.html
COMMIT: 0735bff
CMSSW: CMSSW_12_4_X_2022-03-21-1100/slc7_amd64_gcc10
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week1/cms-sw/cmsdist/7704/23250/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

@slava77 comparisons for the following workflows were not done due to missing matrix map:

• /data/cmsbld/jenkins/workspace/compare-root-files-short-matrix/data/PR-fdfa10/39434.911_TTbar_14TeV+2026D88_DD4hep+TTbar_14TeV_TuneCP5_GenSimHLBeamSpot14+DigiTrigger+RecoGlobal+HARVESTGlobal

Summary:

• No significant changes to the logs found
• Reco comparison results: 2 differences found in the comparisons
• DQMHistoTests: Total files compared: 49
• DQMHistoTests: Total histograms compared: 3695650
• DQMHistoTests: Total failures: 51571
• DQMHistoTests: Total nulls: 70
• DQMHistoTests: Total successes: 3643987
• DQMHistoTests: Total skipped: 22
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.004 KiB( 48 files compared)
• DQMHistoSizes: changed ( 312.0 ): 0.004 KiB MessageLogger/Warnings
• Checked 204 log files, 45 edm output root files, 49 DQM output files
• TriggerResults: no differences found

[smuzaffar]

+externals

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next IB/CMSSW_12_4_X/master IBs (tests are also fine). This pull request will now be reviewed by the release team before it's merged. @perrotta, @dpiparo, @qliphy (and backports should be raised in the release meeting by the corresponding L2)