Contributor notes (http://mail.openjdk.java.net/pipermail/awt-dev/2007-May/000000.html):

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Trying to make OpenJDK more friendly for those distributions (like Gentoo and Debian iirc) having in their policy the use, whenever possible, of the system's libraries rather than internal copies of them, I've prepared three patches to AWT's libsplashscreen that allows to use the external/system copy of the three libraries in the subject.

The default behaviour is unchanged, you need to pass EXTERNAL_JPEG
EXTERNAL_LIBPNG and EXTERNAL_GIFLIB with the value "true" to get them
used.
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See the patches in the Evaluation field.