Makefile: use envsubst for processing .in files

Makefile

@@ -20,16 +20,8 @@ SYSTEMDDIR = $(PREFIX)/lib/systemd/system
 MAN1DIR    = $(PREFIX)/share/man/man1
 MAN5DIR    = $(PREFIX)/share/man/man5
 
-replace_vars = sed \
-	-e "s|@PN@|$(PN)|g" \
-	-e "s|@CONFDIR@|$(CONFDIR)|g" \
-	-e "s|@CRONDIR@|$(CRONDIR)|g" \
-	-e "s|@BINDIR@|$(BINDIR)|g" \
-	-e "s|@DOCDIR@|$(DOCDIR)|g" \
-	-e "s|@SCRIPTDIR@|$(SCRIPTDIR)|g" \
-	-e "s|@SYSTEMDDIR@|$(SYSTEMDDIR)|g" \
-	-e "s|@MAN1DIR@|$(MAN1DIR)|g" \
-	-e "s|@MAN5DIR@|$(MAN5DIR)|g"
+# make variables accessible to `envsubst`
+.EXPORT_ALL_VARIABLES:
 
 all: man
 
@@ -48,8 +40,8 @@ install-etc:
 install-systemd:
 	@echo 'installing systemd service units...'
 	install -d -m 755 "$(DESTDIR)$(SYSTEMDDIR)"
-	$(replace_vars) contrib/systemd/btrbk.service.in > contrib/systemd/btrbk.service.tmp
-	$(replace_vars) contrib/systemd/btrbk.timer.in > contrib/systemd/btrbk.timer.tmp
+	envsubst < contrib/systemd/btrbk.service.in > contrib/systemd/btrbk.service.tmp
+	envsubst < contrib/systemd/btrbk.timer.in > contrib/systemd/btrbk.timer.tmp
 	install -p -m 644 contrib/systemd/btrbk.service.tmp "$(DESTDIR)$(SYSTEMDDIR)/btrbk.service"
 	install -p -m 644 contrib/systemd/btrbk.timer.tmp "$(DESTDIR)$(SYSTEMDDIR)/btrbk.timer"
 	rm contrib/systemd/btrbk.service.tmp

contrib/systemd/btrbk.service.in

@@ -4,4 +4,4 @@ Documentation=man:btrbk(1)
 
 [Service]
 Type=oneshot
-ExecStart=@BINDIR@/btrbk run
+ExecStart=${BINDIR}/btrbk run