[Build] Install the .service file in $(datadir)/dbus-1/services

Calling `pkg-config --variable=session_bus_services_dir dbus-1` under
JHBuild results in the system-installed dbus-1.pc being parsed and
our .service file will be put under the /usr prefix instead of the
JHBuild one.

To avoid that, take $(datadir) in account and install it under
$(datadir)/dbus-1/services by default like other packages do (eg.
gvfs).

For those with different needs a --with-dbus-service-dir configure flag
is provided.
[Build] Use --warning=no-portability for autoreconf

Assuming GNU Make in Automake is a quite safe thing to do given its
availability and enables us to write shorter/simpler rules.
[Build] Do not assume libexec=@prefix@/libexec

For instance, Debian Policy recommends that helper programs should be
installed under /usr/lib/${packagename}/ .

http://www.debian.org/doc/debian-policy/ch-sharedlibs.html#s-sharedlibs-support-files