Stop using "prefix_dir" in libjpeg.gyp.

This reverts part of r257324: prefix_dir is a variable defined only by
FreeBSD's Chromium packaging code and should not be used here upstream.

R=rmcilroy@chromium.org, phajdan.jr, rmcilroy

Review URL: https://codereview.chromium.org/214413007

git-svn-id: http://src.chromium.org/svn/trunk/deps/third_party/libjpeg_turbo@259851 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
1 file changed