hg: jdk8u/jdk8u-dev/langtools: 8217676: Upgrade libpng to 1.6.37

[zgu at redhat.com]

Changeset: 9e6bb52ba514
Author:    zgu
Date:      2019-09-12 15:22 -0400
URL:       https://hg.openjdk.java.net/jdk8u/jdk8u-dev/langtools/rev/9e6bb52ba514

8217676: Upgrade libpng to 1.6.37
Reviewed-by: prr, jdv, kcr

! THIRD_PARTY_README