systemd: add option to enable compatibility libs

Since systemd 209, some libraries have been merged into libsystemd.so:

 - libsystemd-daemon.so
 - libsystemd-id128.so
 - libsystemd-journal.so
 - libsystemd-login.so

A new configuration menu entry has been added to enable the installation
of compatibility pkg-config files, so that programs that depend on them
can still be built.

[Thomas: as suggested by Yann during the review, add an else clause
with --disable-compat-libs.]

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>