[8u] RFR 8220495: Update GIFlib library to the 5.1.8

[Dmitry Cherepanov]

Hello,

Please review the backport of 8220495 to 8u-dev.

JBS: https://bugs.openjdk.java.net/browse/JDK-8220495
Webrevs: http://cr.openjdk.java.net/~dcherepanov/8220495/v0/

The patch applies cleanly after unshuffling except the change in giflib.md. It’s incorporated into THIRD_PARTY_README for all repos.

Testing: build passed on Mac/Win/Linux/Solaris and relevant tests (java/awt/SplashScreen/sun/java2d) didn't reveal any issues.

Thanks,

Dmitry