Add support for DESTDIR

Makefile

@@ -5,8 +5,10 @@ MKDIR=mkdir -p
 all:
 	@echo "Nothing to be done."
 
+test:	check
+
 check:
-	cd tests && make check		# NOTE: May need to run the test with other compilers? clang ? musl-gcc?
+	cd tests && make check
 	cd example && make examples
 
 doc:	README.md
@@ -19,5 +21,5 @@ clean:
 	$(RM) -f README.html
 
 install:
-	$(MKDIR) $(PREFIX)/include
-	cp m-*.h $(PREFIX)/include
+	$(MKDIR) $(DESTDIR)$(PREFIX)/include
+	cp m-*.h $(DESTDIR)$(PREFIX)/include