configure.ac: add check for systemd library and header files

configure.ac: add check for systemd library and header files

systemd support is now enabled by default, unless explicitly disabled
during the ./configure by passing --enable-systemd=no. The make may
fail with:
systemd.c:9:10: fatal error: systemd/sd-bus.h: No such file or directory
    9 | #include <systemd/sd-bus.h>
      |          ^~~~~~~~~~~~~~~~~~
compilation terminated.

if the host system is missing the systemd library and development files,
due to missing checks for their existence in ./configure.ac. This patch
add them.

Fixes: https://github.com/libcgroup/libcgroup/issues/387
Reported-by: Tomasz Kłoczko <kloczek@github.com>
Signed-off-by: Kamalesh Babulal <kamalesh.babulal@oracle.com>
Signed-off-by: Tom Hromatka <tom.hromatka@oracle.com>
(cherry picked from commit 36d1d2226f3f3a089334c7a62a6e5890ae9d52be)