[jdk11u-dev] RFR: 8217633: Configurable extensions with system properties

[Martin Doerr]

Clean backport of JDK-8217633.

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Commit messages:
 - Backport 71bfe962ebcf46537d4624e5ddf36003365fbb84

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/368/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=368&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8217633
  Stats: 145 lines in 2 files changed: 121 ins; 1 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/368.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/368/head:pull/368

PR: https://git.openjdk.java.net/jdk11u-dev/pull/368