Install into /usr/local/ by default

/usr is package manager territory, and not even writable on OSTree based
systems such as Fedora CoreOS or RHEL Edge. This has been common and
good practice in autotools for decades.

You can still override this with `make install PREFIX=...`. Use that in
the RPM spec file.

Makefile

@@ -9,6 +9,7 @@ export TEST_OS
 TARFILE=$(RPM_NAME)-$(VERSION).tar.xz
 NODE_CACHE=$(RPM_NAME)-node-$(VERSION).tar.xz
 SPEC=$(RPM_NAME).spec
+PREFIX ?= /usr/local
 APPSTREAMFILE=org.cockpit-project.$(PACKAGE_NAME).metainfo.xml
 VM_IMAGE=$(CURDIR)/test/images/$(TEST_OS)
 # stamp file to check for node_modules/
@@ -91,12 +92,12 @@ clean:
 	rm -f po/LINGUAS
 
 install: $(WEBPACK_TEST) po/LINGUAS
-	mkdir -p $(DESTDIR)/usr/share/cockpit/$(PACKAGE_NAME)
-	cp -r dist/* $(DESTDIR)/usr/share/cockpit/$(PACKAGE_NAME)
-	mkdir -p $(DESTDIR)/usr/share/metainfo/
+	mkdir -p $(DESTDIR)$(PREFIX)/share/cockpit/$(PACKAGE_NAME)
+	cp -r dist/* $(DESTDIR)$(PREFIX)/share/cockpit/$(PACKAGE_NAME)
+	mkdir -p $(DESTDIR)$(PREFIX)/share/metainfo/
 	msgfmt --xml -d po \
 		--template $(APPSTREAMFILE) \
-		-o $(DESTDIR)/usr/share/metainfo/$(APPSTREAMFILE)
+		-o $(DESTDIR)$(PREFIX)/share/metainfo/$(APPSTREAMFILE)
 
 # this requires a built source tree and avoids having to install anything system-wide
 devel-install: $(WEBPACK_TEST)