Respect `--sysconfdir` in source files

* Prefix installations need to point to a non-root `etc`
  - Gentoo Prefix has been patching this for over 10 years:
    https://bugs.gentoo.org/317891
  - MacPorts has to manually replace paths after patching:
    https://github.com/macports/macports-ports/blob/cc3bb736e906abe73b014da02a89ae2b70ef6295/textproc/libxml2/Portfile#L46