If any autotools-conventional dir names are explicitly overridden via ./configure...

If any autotools-conventional dir names are explicitly overridden via ./configure --dirname=X then export those names as-is to the makefile, otherwise derive them from $prefix. This is, at least in part, a compromise to provide package maintainers a way to specify a libdir which will be incorporated into the -rpath link flag (which we can only determine at configure-time).

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