Disable SCM early if its command is missing

Merged Johann Dreo requested to merge github/fork/dolmen/feature/auto-disable-SCM into develop

Created by: dolmen

LP_ENABLE_{GIT,SVN,HG} is set to 0 if its command is not available in $PATH.

@ldidry Also fixes errors in handling of the LP_ENABLE_{GIT,SVN,HG} variables:

  • Shell functions do not return a string but an error code. 4b42dffb
  • Boolean type does not exist in shell, so we now only handle == 1 or != 1. 7d4ca770

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