fix detection of systemd libs on systemd >= 230

[cjmayo]

There was a mistake in c6773f6 that I missed.

I have tested this change with systemd-226 and systemd-231

[cjmayo]

Updated to quote $SYSTEMD_LIBS to ensure test works.

[langemeijer]

On my stretch build system (systemd-232) I can do aclocal; autoconf; automake; ./configure --with-systemd; make and it will build fine, without your change.

Although configure shows this curious output:

checking for SYSTEMD... no
checking for SYSTEMD... yes

[cjmayo]

Is pkg-config installed?

With this patch configure output should be similar to:

checking for x86_64-pc-linux-gnu-pkg-config... /usr/bin/x86_64-pc-linux-gnu-pkg-config
checking pkg-config is at least version 0.9.0... yes
checking for libsystemd-daemon... no
checking for libsystemd >= 230... yes

And if it has worked near the end:

SYSTEMD:                   -L/lib64 -lsystemd

[langemeijer]

Right! I thought it would work without patch, but indeed your patch is required.

SYSTEMD:                   -lsystemd