RFR: 8366836: Don't execute post-IncludeCustomExtension if file was not included

[Magnus Ihse Bursie]

There is a bug in make/common/MakeIncludeEnd.gmk, which makes it always include the "post" custom extension. In MakeIncludeStart.gmk, the "pre" custom extension is properly guarded by the check if the file has not already been included. This is not the case for the "post" condition, so a custom "post" hook can be included multiple times.

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Commit messages:
 - 8366836: Don't execute post-IncludeCustomExtension if file was not included

Changes: https://git.openjdk.org/jdk/pull/27077/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27077&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366836
  Stats: 10 lines in 2 files changed: 5 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27077.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27077/head:pull/27077

PR: https://git.openjdk.org/jdk/pull/27077