feat(Makefile): GO and SYSTEM_DISTROBOX variables, several improvements

* add GO variable to specify the path to the `go` binary
* add SYSTEM_DISTROBOX variable, and if it's defined, skip trying to install Distrobox
* make `BINARY_NAME` a target, so `make install` only rebuilds if the binary is not present
* assign variables using `:=` (simple expansion) to avoid potential gotchas caused by recursive expansion

Makefile

@@ -1,17 +1,28 @@
-PREFIX=/usr/
-DESTDIR=/
-BINARY_NAME=apx
+PREFIX := /usr
+DESTDIR := /
+BINARY_NAME := apx
+
+GO := go
+SYSTEM_DISTROBOX :=
 
 all: build
 
-build:
-	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build -a -tags netgo -ldflags '-w -extldflags "-static"' -o ${BINARY_NAME}
+build: ${BINARY_NAME}
+
+${BINARY_NAME}:
+	CGO_ENABLED=0 GOOS=linux GOARCH=amd64 ${GO} build -a -tags netgo -ldflags '-w -extldflags "-static"' -o $@
 
+ifdef SYSTEM_DISTROBOX
 install: build
+else
+install: build install-distrobox
+endif
 	install -Dm755 ${BINARY_NAME} ${DESTDIR}${PREFIX}/bin/${BINARY_NAME}
 	mkdir -p ${DESTDIR}/etc/apx
-	sed -i 's|/usr/share/apx/distrobox|${PREFIX}/share/apx/distrobox|g' config/apx.json
+	sed -i 's|/usr/share/apx/distrobox|${DESTDIR}${PREFIX}/share/apx/distrobox|g' config/apx.json
 	install -Dm644 config/apx.json ${DESTDIR}/etc/apx/apx.json
+
+install-distrobox:
 	mkdir -p ${DESTDIR}${PREFIX}/share/apx/distrobox
 	sh distrobox/install --prefix ${DESTDIR}${PREFIX}/share/apx/distrobox
 	mv ${DESTDIR}${PREFIX}/share/apx/distrobox/bin/distrobox* ${DESTDIR}${PREFIX}/share/apx/distrobox/.
@@ -22,7 +33,7 @@ install-manpages:
 	chmod 644 ${DESTDIR}${PREFIX}/share/man/man1/apx*
 
 uninstall: uninstall-manpages
-	rm ${DESTDIR}${PREFIX}/bin/apx
+	rm ${DESTDIR}${PREFIX}/bin/${BINARY_NAME}
 	rm -rf ${DESTDIR}/etc/apx
 	rm -rf ${DESTDIR}${PREFIX}/share/apx
 
@@ -31,4 +42,4 @@ uninstall-manpages:
 
 clean:
 	rm -f ${BINARY_NAME}
-	go clean
+	${GO} clean