Namely: no, yes, and yes and ask at startup.
The install_check option is thus replaced by the can_install option. -Dinstall_check=true maps to -Dcan_install=yes_check, while -Dinstall_check=false maps to -Dcan_install=yes. -Dcan_install=no is new and is recommended for downstream packaging, where -Dinstall_check=false was historically used.
Also improve error messages about bad configuration here and there and scatter configuration code in subdirectories, where they can be closer to their areas of effect.
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