systemd: fall back to default system-sleep dir without systemd-devel

Keep the pkg-config lookup for SYSTEMD_SLEEP_DIR, but fall back to
"$(prefix)/lib/systemd/system-sleep/" if the variable remains unset.
This avoids reintroducing a build-time dependency on systemd-devel while
still honoring the path provided by pkg-config when present.

Fixes: e54a6cd6a1 ("systemd: Add hooks to stop/start xen-watchdog on suspend/resume")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Mykola Kvach <mykola_kvach@epam.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit b34bde262c)

m4/systemd.m4

@@ -81,7 +81,7 @@ AC_DEFUN([AX_CHECK_SYSTEMD_LIBS], [
 	], [])
 
 	AS_IF([test "x$SYSTEMD_SLEEP_DIR" = x], [
-	    AC_MSG_ERROR([SYSTEMD_SLEEP_DIR is unset])
+	    SYSTEMD_SLEEP_DIR="\$(prefix)/lib/systemd/system-sleep/"
 	], [])
 ])
 

tools/configure

@@ -10360,7 +10360,7 @@ fi
 	if test "x$SYSTEMD_SLEEP_DIR" = x
 then :
 
-	    as_fn_error $? "SYSTEMD_SLEEP_DIR is unset" "$LINENO" 5
+	    SYSTEMD_SLEEP_DIR="\$(prefix)/lib/systemd/system-sleep/"
 
 fi