fix: Use DESTDIR properly in the Makefile and introduce PREFIX

This allows for packaging tooling to use the Makefile properly.

Makefile

@@ -1,17 +1,18 @@
 export CARGO_TARGET_DIR ?= ./target
-DESTDIR ?= /usr/local
+DESTDIR ?= /
+PREFIX ?= /usr/local
 
 build-release:
 	cargo build --release
 
 install:
-	install -Dm755 target/release/lact ${DESTDIR}/bin/lact
-	install -Dm644 res/lactd.service ${DESTDIR}/lib/systemd/system/lactd.service
-	install -Dm755 res/io.github.lact-linux.desktop ${DESTDIR}/share/applications/io.github.lact-linux.desktop
-	install -Dm755 res/io.github.lact-linux.png ${DESTDIR}/share/pixmaps/io.github.lact-linux.png
+	install -Dm755 target/release/lact $(DESTDIR)$(PREFIX)/bin/lact
+	install -Dm644 res/lactd.service $(DESTDIR)$(PREFIX)/lib/systemd/system/lactd.service
+	install -Dm755 res/io.github.lact-linux.desktop $(DESTDIR)$(PREFIX)/share/applications/io.github.lact-linux.desktop
+	install -Dm755 res/io.github.lact-linux.png $(DESTDIR)$(PREFIX)/share/pixmaps/io.github.lact-linux.png
 
 uninstall:
-	rm ${DESTDIR}/bin/lact
-	rm ${DESTDIR}/lib/systemd/system/lactd.service
-	rm ${DESTDIR}/share/applications/io.github.lact-linux.desktop
-	rm ${DESTDIR}/share/pixmaps/io.github.lact-linux.png
+	rm $(DESTDIR)$(PREFIX)/bin/lact
+	rm $(DESTDIR)$(PREFIX)/lib/systemd/system/lactd.service
+	rm $(DESTDIR)$(PREFIX)/share/applications/io.github.lact-linux.desktop
+	rm $(DESTDIR)$(PREFIX)/share/pixmaps/io.github.lact-linux.png