This is needed to build libFLAC which uses ftello().

The `dump` command was dropped[1] in favor of other tools.

[1]: https://gitlab.freedesktop.org/pipewire/pipewire/-/commit/50bdebe4e87c5e26acf966e7207a591332e33239

systemd uses "rootprefix", not "prefix" for this value

https://github.com/systemd/systemd/blob/059b1b31ade7f1d716f56c5e6fd657ce22ce2032/src/core/systemd.pc.in#L23

See https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/

Fixes the build in nixpkgs

6d91b5c7 ("fix double promotions") changed
how LAME peak values are decoded, producing large incorrect values that
cause some MP3 files to play silently.

Restore the original decode from MAD fixed-point format to double and
document what it's doing.

Fixes #1823

With sphinx-build 5.0.0:
doc/user.rst:728: ERROR: Unexpected indentation.
doc/user.rst:731: ERROR: Unexpected indentation.