inside a subdirectory of the working directory, instead of elsewhere.
Allows packages BULK=Yes to work correctly, and increase reproducibility
of builds.
tested by naddy@ through several incarnations.
Ports that use patch/configure/build dependencies must be adjusted for this.
Pleasingly enough, the adjusted version is often simpler.
The change uses an internal _MASTER variable to recall the list of ports
built during dependencies, for less confusion.
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