[jdk11u-dev] RFR: 8296904: Improve handling of macos xcode toolchain

[Christoph Langer]

This is a backport of JDK-8296904. Changes to make/autoconf/basic.m4 from jdk17u-dev apply cleanly.
I resolved building.md to the state of jdk11u-dev and regenerated the html files with Pandoc 2.3.1. This also modified testing.html.

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Commit messages:
 - Backport 57e6d92a6ddbfe78cbf250a4502ce0911ee6f75a

Changes: https://git.openjdk.org/jdk11u-dev/pull/1584/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk11u-dev&pr=1584&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296904
  Stats: 91 lines in 4 files changed: 24 ins; 1 del; 66 mod
  Patch: https://git.openjdk.org/jdk11u-dev/pull/1584.diff
  Fetch: git fetch https://git.openjdk.org/jdk11u-dev pull/1584/head:pull/1584

PR: https://git.openjdk.org/jdk11u-dev/pull/1584