Use standard variable names for install rules

As per
https://www.gnu.org/software/make/manual/html_node/Directory-Variables.html

The INSTALL_PREFIX variable name has been deprecated in favour of
DESTDIR, so having both of them in the rule didn't make sense.

We now default to /usr/local as the install prefix, as recommended by
the GNU Make manual.

Author: torarnv

Makefile

@@ -194,13 +194,17 @@ clean:
 distclean: clean
 	rm -Rf $(TESTDATA_DIR)
 
+# Install
+
+prefix = /usr/local
+
 .PHONY: install
 install: sparsebundlefs
-	install -d "$(DESTDIR)$(INSTALL_PREFIX)/bin"
-	install -m 755 sparsebundlefs "$(DESTDIR)$(INSTALL_PREFIX)/bin/"
+	install -d "$(DESTDIR)$(prefix)/bin"
+	install -m 755 sparsebundlefs "$(DESTDIR)$(prefix)/bin/"
 
 .PHONY: uninstall
 uninstall:
-	rm -f $(DESTDIR)$(PREFIX)/bin/sparsebundlefs
+	rm -f $(DESTDIR)$(prefix)/bin/sparsebundlefs
 
 endif