src/Makefile: compile systemd support by default

src/Makefile: compile systemd support by default

Compile systemd support file by default, now we have stub and defined
versions of the systemd functions for both non-systemd and systemd
enabled configurations.  Pass linker flag WITH_SYSTEMD only when
--enable-systemd is set, that will call the defined function, otherwise
call the stub functions.

Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>