configure: Refactor systemunitdir expression to use PKG_CHECK_VAR

Change-Id: Ia78044b47d2e01ec2629ba2bc7d8033c0107afb9
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/bmc/firmware-handler/Makefile.am b/bmc/firmware-handler/Makefile.am
index f4d8e43..c12838d 100644
--- a/bmc/firmware-handler/Makefile.am
+++ b/bmc/firmware-handler/Makefile.am
@@ -25,7 +25,7 @@
 dist_pkgdata_DATA += config-bios.json
 endif
 
-if HAVE_SYSTEMD
+if HAVE_SYSTEMUNITDIR
 systemdsystemunit_DATA = \
 	phosphor-ipmi-flash-bmc-prepare.target \
 	phosphor-ipmi-flash-bmc-verify.target \
@@ -37,6 +37,7 @@
 	phosphor-ipmi-flash-bios-update.target
 endif
 endif
+
 # Convenience libraries that are discarded after build
 noinst_LTLIBRARIES = libfirmwareblob_common.la