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This is done by VLC already, and was cluttering our build environment by
preventing contribs to be used when a package is already present on the
system (even though the version doesn't match our requirement)

This reverts commit 90c43e9b.
It is used by vlc's contrib bootstrap

Since the compilation has been fixed, no need to stick to an old libpng
version

Instead, explicitely set MAKE_JOBS environment variable to 1 if you want
this behavior.