A few remaining variables in spec.gmk, like `BUILD_JDK`, is setup by configure to include makefile variables, like `$(JDK_OUTPUTDIR)`.

This makes reasoning about spec.gmk unnecessary complex. Instead, we should just expose simple, resolved text strings from configure. Any remaining logic is better expressed directly in the makefiles.

This is part of an effort to clean up spec.gmk to not contain so much weird stuff.