Rewrite makefile to use cargo install

This makes gir build work regardless of any `target-dir` overrides.

[skip ci]

Makefile

@@ -1,14 +1,17 @@
-GIR = gir/target/release/gir
-GTK_GIR = gir-files/Gtk-3.0.gir
+GIR = gir/target/bin/gir
+GIR_SRC = gir/Cargo.toml gir/Cargo.lock gir/build.rs $(shell find gir/src -name '*.rs')
+GIR_FILES = gir-files/Gtk-3.0.gir
 
 # Run `gir` generating the bindings
 gir : src/auto/mod.rs
 
-src/auto/mod.rs : Gir.toml $(GIR) $(GTK_GIR)
+src/auto/mod.rs : Gir.toml $(GIR) $(GIR_FILES)
 	$(GIR) -c Gir.toml
 
-$(GIR) : gir/Cargo.toml gir/Cargo.lock gir/build.rs $(shell find gir/src -name '*.rs')
-	cd gir && cargo build --release
+$(GIR) : $(GIR_SRC)
+	rm -f gir/target/bin/gir
+	cargo install --path gir --root gir/target
+	rm -f gir/target/.crates.toml
 
-gir/Cargo.toml $(GTK_GIR) :
+$(GIR_SRC) $(GIR_FILES) :
 	git submodule update --init