fixed make vars to be consistent with other lib makefiles

Makefile

@@ -1,6 +1,8 @@
 HDRS := $(wildcard *.hpp)
-PREFIX ?= /usr/local/include
+
+PREFIX ?= /usr/local
+INCLUDEDIR ?= $(PREFIX)/include
 
 install: $(HDRS)
-	mkdir -p $(PREFIX)/sc
-	install -c -m 0666 $(HDRS) $(PREFIX)/sc
+	mkdir -p $(INCLUDEDIR)/sc
+	install -c -m 0666 $(HDRS) $(INCLUDEDIR)/sc