Makefile: factor in PREFIX to install directories

Sometimes we want to set PREFIX and leave the installation directories relative
to the PREFIX.

Other times we want to set some of these installation directories to absolute
values.

Sometimes we want both.

This commit patch achieves this with a reasonably small effort, by not
propagating PREFIX around the Makefile.

Makefile

@@ -9,10 +9,10 @@ CFLAGS += -Wall -Wextra -Wformat-security -Wconversion -DVERSION=\"$(VERSION)\"
 
 DESTDIR ?=
 PREFIX ?= /usr/local
-BINDIR ?= /bin
-MANDIR ?= /share/man
+BINDIR ?= $(PREFIX)/bin
+MANDIR ?= $(PREFIX)/share/man
 MAN1DIR ?= $(MANDIR)/man1
-SYSCONFDIR ?= /etc
+SYSCONFDIR ?= $(PREFIX)/etc
 SYSTEMDUNITDIR ?= $(SYSCONFDIR)/systemd/system
 PANDOC := $(shell command -v pandoc 2> /dev/null)
 
@@ -49,20 +49,20 @@ install-initscript: earlyoom.initscript install-bin install-default
 	-update-rc.d earlyoom start 18 2 3 4 5 . stop 20 0 1 6 .
 
 earlyoom.%: earlyoom.%.in
-	sed "s|:TARGET:|$(PREFIX)$(BINDIR)|g;s|:SYSCONFDIR:|$(SYSCONFDIR)|g" $< > $@
+	sed "s|:TARGET:|$(BINDIR)|g;s|:SYSCONFDIR:|$(SYSCONFDIR)|g" $< > $@
 
 install-default: earlyoom.default install-man
 	install -d $(DESTDIR)$(SYSCONFDIR)/default/
 	install -m 644 $< $(DESTDIR)$(SYSCONFDIR)/default/earlyoom
 
 install-bin: earlyoom
-	install -d $(DESTDIR)$(PREFIX)$(BINDIR)/
-	install -m 755 $< $(DESTDIR)$(PREFIX)$(BINDIR)/
+	install -d $(DESTDIR)$(BINDIR)/
+	install -m 755 $< $(DESTDIR)$(BINDIR)/
 
 install-man: earlyoom.1.gz
 ifdef PANDOC
-	install -d $(DESTDIR) $(PREFIX)$(MAN1DIR)/
-	install -m 644 $< $(DESTDIR)$(PREFIX)$(MAN1DIR)/
+	install -d $(DESTDIR) $(MAN1DIR)/
+	install -m 644 $< $(DESTDIR)$(MAN1DIR)/
 endif
 
 earlyoom.1.gz: earlyoom.1
@@ -75,14 +75,14 @@ uninstall: uninstall-bin uninstall-man
 	rm -f $(DESTDIR)$(SYSTEMDUNITDIR)/earlyoom.service
 
 uninstall-man:
-	rm -f $(DESTDIR)$(PREFIX)$(MAN1DIR)/earlyoom.1.gz
+	rm -f $(DESTDIR)$(MAN1DIR)/earlyoom.1.gz
 
 uninstall-initscript: uninstall-bin
 	rm -f $(DESTDIR)$(SYSCONFDIR)/init.d/earlyoom
 	update-rc.d earlyoom remove
 
 uninstall-bin:
-	rm -f $(DESTDIR)$(PREFIX)$(BINDIR)/earlyoom
+	rm -f $(DESTDIR)$(BINDIR)/earlyoom
 
 # Depends on earlyoom compilation to make sure the syntax is ok.
 format: earlyoom