The build directory is meaningless, since it is typically some
directory in a continuous integration server. That means someone
debugging the relocatable package needs to issue the gdb command
'set substitute-path' with the correct arguments, or they lose
source debugging. Doing so in the relocatable package build saves
this step.
The default build is not modified, since a typical local build
benefits from having the paths hardcoded, as the debugger will
find the sources automatically.
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