Support installation (#63)

Seems reasonable

Author: AMDmi3

Makefile

@@ -1,9 +1,12 @@
+PREFIX?=	/usr/local
+
 help:
 	@echo This makefile is just for the test program \(use \"make clean all test\"\)
 	@echo Just include the SimpleIni.h header file to use it.
 
 install:
-	@echo No install required. Just include the SimpleIni.h header file to use it.
+	mkdir -p $(DESTDIR)$(PREFIX)/include/
+	install -C -m 644 SimpleIni.h $(DESTDIR)$(PREFIX)/include/
 
 TOPTARGETS := all clean test