[8u] RFR: 8230597: Update GIFlib library to the 5.2.1

[Severin Gehwolf]

Hi,

Could I get a review of this 8u backport of updating giflib to version
5.2.1? This is an Oracle JDK 251 feature parity patch. The JDK 11u
patch applies clean modulo giflib.md changes, which is in
THIRD_PARTY_README in 8u.

THIRD_PARTY_README changes have been done by hand. Once approved, I
intend to push THIRD_PARTY_README changes to all 7 other repos as well.
Note that the Copyright header in 8u already has the double space after
'1997'[1]. Also the copyright changes related to openbsd-reallocarray.c 
have been dropped since this isn't beeing mentioned in
THIRD_PARTY_README at all.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230597
webrev: http://cr.openjdk.java.net/~sgehwolf/webrevs/JDK-8230597/01/webrev/

Testing: Smoke testing splash screen demos with gifs.

Thoughts?

Thanks,
Severin

[1] https://hg.openjdk.java.net/jdk-updates/jdk11u/rev/ac6ef7c537f7#l1.11
    JDK 8u has: 
    http://hg.openjdk.java.net/jdk8u/jdk8u-dev/jdk/file/fd05eae58d9c/THIRD_PARTY_README#l1651