Systemd directory to install unit files.

Added a new systemd directory to hold & install unit files
appropriately under prefix/lib/systemd/system folder. Systemd
unit files are installed only if the host machine has support
for it.
Add pkg-config check for systemd.

Changed configure.ac to add pkg-config test to see if systemd exists
or not. When it does, AM_CONDITIONAL defines a variable, which is then
used to conditionally build & install systemd unit files or init.d
script files.