Use BINDIR, MANDIR and SYSCONFDIR vars in Makefile

This commit introduces these variables, and defaults to using
the same values as the ones that were used before. The advantage
of doing this is to make it easier to use some distro-specific
layout. When this is desirable, these variables can be overridden
to what is appropriate at package build time.

Makefile

 PREFIX ?= /usr
 DESTDIR ?=
+BINDIR ?= $(PREFIX)/bin
+MANDIR ?= $(PREFIX)/share/man
+SYSCONFDIR ?= /etc
 
 .PHONY: install
 
@@ -7,10 +10,10 @@ all:
 	@echo "Password store is a shell script, so there is nothing to do. Try \"make install\" instead."
 
 install:
-	@mkdir -p $(DESTDIR)$(PREFIX)/bin $(DESTDIR)$(PREFIX)/share/man/man1 $(DESTDIR)/etc/bash_completion.d
-	@install -v src/password-store.sh $(DESTDIR)$(PREFIX)/bin/pass
-	@install -v man/pass.1 $(DESTDIR)$(PREFIX)/share/man/man1/pass.1
-	@install -v bash-completion/pass-bash-completion.sh $(DESTDIR)/etc/bash_completion.d/password-store
+	@mkdir -p $(DESTDIR)$(BINDIR) $(DESTDIR)$(MANDIR)/man1 $(DESTDIR)$(SYSCONFDIR)/bash_completion.d
+	@install -v src/password-store.sh $(DESTDIR)$(BINDIR)/pass
+	@install -v man/pass.1 $(DESTDIR)$(MANDIR)/man1/pass.1
+	@install -v bash-completion/pass-bash-completion.sh $(DESTDIR)$(SYSCONFDIR)/bash_completion.d/password-store
 
 uninstall:
-	@rm -vf $(DESTDIR)$(PREFIX)/bin/pass $(DESTDIR)$(PREFIX)/share/man/man1/pass.1 $(DESTDIR)/etc/bash_completion.d/password-store
+	@rm -vf $(DESTDIR)$(BINDIR)/pass $(DESTDIR)$(MANDIR)/man1/pass.1 $(DESTDIR)$(SYSCONFDIR)/bash_completion.d/password-store