Compile plugins with -fno-gnu-unique.

plugin.mk

@@ -16,6 +16,9 @@ DISTRIBUTABLES += plugin.json
 
 FLAGS += -fPIC
 FLAGS += -I$(RACK_DIR)/include -I$(RACK_DIR)/dep/include
+# This prevents static variables in the DSO (dynamic shared object) from being preserved after dlclose().
+# I don't really understand the side effects (see GCC manual), but so far tests are positive.
+FLAGS += -fno-gnu-unique
 
 include $(RACK_DIR)/arch.mk