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

Port of commits:
https://github.com/01org/dleyna-renderer/commit/d1c84c820ce180eabb57b83da3569a4de24c3ac4
https://github.com/01org/dleyna-renderer/commit/6981acf20cadec8a9c29e0096d16c2062d19a384

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.

Signed-off-by: Regis Merlino <regis.merlino@intel.com>
[Build] Do not assume libexec=@prefix@/libexec

This is a port of commits:
https://github.com/01org/dleyna-renderer/commit/8179084be7b32f187e9ab4d79a24efa668a6df71
https://github.com/01org/dleyna-renderer/commit/f3e9967b3c8f76b6d43320749b6e58e1a607d2a9

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

Signed-off-by: Regis Merlino <regis.merlino@intel.com>
[Build] Do not require dbus and dleyna-connector-dbus at build time

This is a port of commit:
https://github.com/01org/dleyna-renderer/commit/efa7cb33e68801059ddbadcd95df46bbae865eb4

DBus and the DBus connector are only needed at runtime and dropping the
pkg-config checks in ./configure allows the build to succeed even in
restricted build environment with minimal dependecies.

Signed-off-by: Regis Merlino <regis.merlino@intel.com>