plugins/c_src.mk: Deduce directory to create from $@

... instead of hard-coding `priv`.

plugins/c_src.mk

@@ -98,7 +98,7 @@ app:: $(C_SRC_ENV) $(C_SRC_OUTPUT_FILE)
 test-build:: $(C_SRC_ENV) $(C_SRC_OUTPUT_FILE)
 
 $(C_SRC_OUTPUT_FILE): $(OBJECTS)
-	$(verbose) mkdir -p priv/
+	$(verbose) mkdir -p $(dir $@)
 	$(link_verbose) $(CC) $(OBJECTS) \
 		$(LDFLAGS) $(if $(filter $(C_SRC_TYPE),shared),-shared) $(LDLIBS) \
 		-o $(C_SRC_OUTPUT_FILE)