build: ensure sdbus++ exists

Change-Id: Ia8b7c676846fb197337fba46e79717744118194a
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/generate_makefile.sh b/generate_makefile.sh
index 4816919..4c103f3 100755
--- a/generate_makefile.sh
+++ b/generate_makefile.sh
@@ -13,11 +13,11 @@
 
 ${i%.interface.yaml}/server.cpp: ${i} ${i%.interface.yaml}/server.hpp
 	@mkdir -p \`dirname \$@\`
-	sdbus++ -r \$(srcdir) interface server-cpp ${iface} > \$@
+	\$(SDBUSPLUSPLUS) -r \$(srcdir) interface server-cpp ${iface} > \$@
 
 ${i%.interface.yaml}/server.hpp: ${i}
 	@mkdir -p \`dirname \$@\`
-	sdbus++ -r \$(srcdir) interface server-header ${iface} > \$@
+	\$(SDBUSPLUSPLUS) -r \$(srcdir) interface server-header ${iface} > \$@
 
 MAKEFILE