Do not link against libdl on OpenBSD

Makefile

@@ -39,7 +39,15 @@ else
 	CXXFLAGS += -D NDEBUG -O3
 endif
 
-LDLIBS := -lm -lscgui -lSDL2 -lcairo -lscerror -lscstring -lscscreensaver -ldl -lscnumerics
+LDLIBS := -lm -lscgui -lSDL2 -lcairo -lscerror -lscstring -lscscreensaver -lscnumerics
+UNAME_S := $(shell uname -s)
+
+ifeq ($(UNAME_S),Darwin)
+	LDLIBS += -ldl
+endif
+ifeq ($(UNAME_S),Linux)
+	LDLIBS += -ldl
+endif
 
 .PHONY: all clean install modules new