Conditional configuration sections: compute values at most once, add 'multiarch'

For example, existing values were not enough to distinguish
'arm-linux-gnueabi' from 'arm-linux-gnueabihf'.

The 'multiarch' value is the output of

  $CC -dumpmachine

where CC defaults to 'gcc'.

See also https://wiki.debian.org/Multiarch/Tuples
14 jobs for master in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed BUILDOUT-NEXT

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passed SlapOS.Buildout.UnitTest-Master.Python2

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passed SlapOS.Buildout.UnitTest-Master.Python3

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passed BUILDOUT-NEXT

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passed BUILDOUT-NEXT

00:16:49

passed SlapOS.Buildout.UnitTest-Master.Python2

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passed SlapOS.Buildout.UnitTest-Master.Python2

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passed SlapOS.Buildout.UnitTest-Master.Python2

01:26:50

passed SlapOS.Buildout.UnitTest-Master.Python2

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passed SlapOS.Buildout.UnitTest-Master.Python2

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passed SlapOS.Buildout.UnitTest-Master.Python3

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passed SlapOS.Buildout.UnitTest-Master.Python3

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passed SlapOS.Buildout.UnitTest-Master.Python3

00:21:29

passed SlapOS.Buildout.UnitTest-Master.Python3

01:47:23