build-sys: fix sysvcompat handling in distcheck

build-sys: fix sysvcompat handling in distcheck

Distcheck would fail due to sysvinit dir being set,
but not sysvrcnd dir:

 # ./configure --enable-gtk-doc --with-sysvrcd-path=/etc/rc.d
 # --with-sysvinit-path=/etc/rc.d
 # make distcheck
 ...
 configure: error: *** You need both --with-sysvinit-path=PATH and
--with-sysvrcd-path=PATH to enable SysV compatibility support, or both
empty to disable it.
make: *** [distcheck] Error 1

This also allows sysvcompat support to be disabled from distcheck.