Documentation: Provide instructions for compiling MET with the C++11 standard

[jprestop]

Describe the Task

Due to the issue Modify configure.ac to require the C++17 standard #2948, provide instructions for users can compile MET with the C++11 standard.

Update the screenshot in this section to include the newer versions of ecKit and ATLAS.

Review the rest of the page for any other necessary changes.

Time Estimate

1-3 days of work

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Relevant Deadlines

MET-12.0.0-rc1 release

Funding Source

TBD

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• METplus, MET, METdataio, METviewer, METexpress, METcalcpy, METplotpy

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[JohnHalleyGotway]

Based on brainstorming with @j-opatz in Slack, recommend taking the following approach.

1. Add support in the MET configure.ac file for a new environment variable named MET_CXX_STANDARD (or something similar).
2. Update configure.ac to add MET_CXX_STANDARD it to the list of "influential env vars" along with a description of how it works.
3. Update configure.ac to check to see if it's set. If so, set a local variable to the same value. If not, set that local variable to the default value of 17.
4. Update configure.ac so that the CXXFLAGS setting includes -std=c++${MET_CXX_STANDARD}... or something similar to specify the C++ standard flag that should be used.