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

[Thomas Devoogdt]

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

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Commit messages:
 - 8376684: doc: fix X11 description when compiling headless
 - 8376684: autoconf: libraries: drop the need for X11 in headless mode
 - 8376684: awt: fix HEADLESS compilation without X11

Changes: https://git.openjdk.org/jdk/pull/28310/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28310&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376684
  Stats: 30 lines in 5 files changed: 14 ins; 5 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/28310.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28310/head:pull/28310

PR: https://git.openjdk.org/jdk/pull/28310