RFR: Allow using the system libjpeg

[Omair Majid]

Hi,

Webrevs are at:
http://cr.openjdk.java.net/~omajid/webrevs/system-libjpeg/00
http://cr.openjdk.java.net/~omajid/webrevs/system-libjpeg/00.jdk

This patch modifies OpenJDK to allow building and running using the
system copy of libjpeg. This follows my previous changes for lcms
and a lot of the decisions made on that patch apply here.

This is the last of my system-library patches :)

Thanks,
Omair

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