build-sys: do not use pkgconfig dbus-1.pc to find dbus directories

Do not use the dbus-1.pc pkgconfig settings to determine dbus directories. Use
directories relative to ${sysconfdir} and ${datadir} instead.

This approach was suggested by Simon McVittie in:
http://lists.freedesktop.org/archives/systemd-devel/2014-October/024388.html

Tested by building and installing systemd without the dbus-devel installed.
Without this patch, the dbus files and directories end up in the root of the
filesystem. With this patch, they end up in the same locations as previously
(assuming default ${sysconfdir} and ${datadir}) whether dbus-devel is present
or not. Also made sure that `make check` works without dbus-devel installed.