RFR: 8368326: Don't export unresolved make variables from configure

[Erik Joelsson]

On Mon, 22 Sep 2025 18:41:11 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> 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.

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27435#pullrequestreview-3255297002