Only ask pkg-config for the systemd unit directory if both are installed

pkg-config is not a hard dependency, and this check avoids errors in a
minimal build environment:

./configure: line 21777: --variable=systemdsystemunitdir: command not found

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=37847Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>

configure.ac

@@ -1373,7 +1373,12 @@ AM_CONDITIONAL(DBUS_INIT_SCRIPTS_CYGWIN, test x$with_init_scripts = xcygwin)
 ##### systemd unit files
 AC_ARG_WITH([systemdsystemunitdir],
 AS_HELP_STRING([--with-systemdsystemunitdir=DIR], [Directory for systemd service files]),
-        [], [with_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)])
+    [],
+    [
+    PKG_CHECK_EXISTS([systemd],
+      [with_systemdsystemunitdir=$($PKG_CONFIG --variable=systemdsystemunitdir systemd)],
+      [with_systemdsystemunitdir=no])
+    ])
 if test "x$with_systemdsystemunitdir" != xno; then
    AC_SUBST([systemdsystemunitdir], [$with_systemdsystemunitdir])
 fi