cmake: Allow overriding SYSCONFDIR for CMake build on unix.

This adds a new CMake option SYSCONFDIR. If SYSCONFDIR is specified then it will be used via the SYSCONFDIR define, and /etc will not be automatically added via EXTRASYSCONFDIR. If SYSCONFDIR is not specified then the old logic will be used (CMAKE_INSTALL_FULL_SYSCONFDIR added via SYSCONFDIR, and /etc, if it's not the same, added via EXTRASYSCONFDIR).