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This makes it easy to know which things will be enabled or
disabled, depending which deps were found or not found.

This rewritten configure.ac is nearly independent of any of the xiph m4
macros, most of which are ancient and can nowadays easily replaced
with pkg-config.

Thanks to ePirat for the suggestion.

Thanks to ePirat.

We should first shut down TLS so we can signal the other end a clean shutdown
in contrast to a socket failure.

Big thanks to wget(1) to find it. Fantastic tool.

This generates error pages based on IDs. This allows to reuse errors
and add more advanced information to them.

This patch also makes Icecast send in plain text OR HTML based
on the clients Accept:-string.