Try to find settings.conf from SYSCONFDIR

Honor SYSCONFDIR as specified by CMake arguments instead of
using a hard-coded path.

Since GNUInstallDirs set CMAKE_INSTALL_FULL_SYSCONFDIR to
CMAKE_INSTALL_PREFIX/etc we need to change the default ourselves.
This won't do because CMAKE_INSTALL_SYSCONFDIR is always defined and by default set to "etc" (relative to the install prefix).