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

Mon Sep 22 19:33:24 UTC 2025

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.

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Commit messages:
 - 8368326: Don't export unresolved make variables from configure

Changes: https://git.openjdk.org/jdk/pull/27435/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368326
  Stats: 54 lines in 2 files changed: 16 ins; 29 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/27435.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27435/head:pull/27435

PR: https://git.openjdk.org/jdk/pull/27435