cmake: add support for systemdsystemunitdir

I have no idea how I missed this:
problem: install-only builds passed `--with-systemdsystemunitdir` which
was not supported in the cmake wrapper and caused a failure

solution: add support for accepting the option.  It currently doesn't
*do* anything, but it's pulled in at least so we can use it when and if
we want to.

.configure-custom.sh

@@ -5,3 +5,5 @@ DISABLE_VARS="docs|Off|ENABLE_DOCS"
 ENABLE_ENABLE_COVERAGE_DOC="enable coverage analysis"
 ENABLE_ENABLE_DOCS_DOC="enable building manpages"
 ENABLE_DISABLE_DOCS_DOC="disable building manpages"
+WITH_VARS="systemdsystemunitdir|/etc/systemd/system|FLUX_SYSTEMD_SYSTEM_UNIT_DIR"
+WITH_FLUX_SYSTEMD_SYSTEM_UNIT_DIR_DOC="systemd system directory"