RFR: 8376684: Compile OpenJDK in headless mode without required X11 libraries

[Alexey Ivanov]

On Thu, 13 Nov 2025 22:17:23 GMT, Thomas Devoogdt <duke at openjdk.org> wrote:

> This to support linux headless mode without X11. E.g. A headless server.

I tested it in a live Ubuntu image. I installed all the dependencies except for X11 headers. The regular run of `configure` failed for me complaining about the missing X11 headers. When I passed the `--enable-headless-only` option to `configure`, it succeeded, and the build succeeded.

I ran headless AWT tests, and all the tests passed.

The fix for [JDK-8255785](https://bugs.openjdk.org/browse/JDK-8255785) that enabled building headless JDK without the X11 headers was reverted by [JDK-8258465](https://bugs.openjdk.org/browse/JDK-8258465) because there was a dependency on definition of the rectangle structure.

The fix proposed in this PR removes that dependency by providing an alternative definition, the same one that's already used on macOS. Thus, this fix also resolves [JDK-8273258](https://bugs.openjdk.org/browse/JDK-8273258).

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Marked as reviewed by aivanov (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28310#pullrequestreview-3747431493