[jdk8u-dev] RFR: 8305815: Update Libpng to 1.6.39

[Martin Balao]

Hi all,

This pull request contains a backport of commit [c1f759e9d01d646eac69442452151b0467eab306](https://github.com/openjdk/jdk11u-dev/commit/c1f759e9d01d646eac69442452151b0467eab306) from the [openjdk/jdk11u-dev](https://github.com/openjdk/jdk11u-dev) repository.

This backport is a follow up fix of [JDK-8295685](https://bugs.openjdk.org/browse/JDK-8295685).

This backport applied cleanly, except for the changes to the THIRD_PARTY_README files that are specific to 8u.

Thanks!

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Depends on: https://git.openjdk.org/jdk8u-dev/pull/372

Commit messages:
 - Backport c1f759e9d01d646eac69442452151b0467eab306

Changes: https://git.openjdk.org/jdk8u-dev/pull/373/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=373&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305815
  Stats: 129 lines in 17 files changed: 29 ins; 8 del; 92 mod
  Patch: https://git.openjdk.org/jdk8u-dev/pull/373.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev.git pull/373/head:pull/373

PR: https://git.openjdk.org/jdk8u-dev/pull/373