Fixes formatted paths in the `recorder` output, closing #345.

Closes #360

Use `const_cast` to be able to pass it to sf_open_virtual() which has
a wrong parameter declaration.

So long, autotools!  This is my last MPD related project to migrate
away from it.  It has its strengths, but also very obvious weaknesses
and weirdnesses.  Today, many of its quirks are not needed anymore,
and are cumbersome and slow.  Now welcome our new Meson overlords!

Broken since commit 08f77c2b

This is the version in Debian Jessie (oldstable).

This is kind of a revert of commit
b2e3c075, which is not any longer
necessary since commit 0dd4b52b
removed the last call to `av_open_input_stream()`.

Remove another dependency on the config/Global library.

This fixes an old bug which caused the "unused" warnings to be
unreliable; only the first block in the list was marked as being
"used", no matter if it was really used, and the rest was never marked
as "used", suppressing all warnings for them.