build(make): follow GNU install dir conventions

Makefile

@@ -1,20 +1,45 @@
-PREFIX := /usr/local
+SHELL = /bin/sh
 
-./agenda.1.gz: ./agenda.1
-	gzip -fk ./agenda.1
+# See the following link regarding GNU Makefile conventions.
+# https://www.gnu.org/software/make/manual/html_node/Makefile-Conventions.html
+
+# This seems stupid. Isn't install a coreutil anyways?
+# INSTALL := install
+# INSTALL_PROGRAM := $(INSTALL)
+# INSTALL_DATA := ${INSTALL} -m 644
+
+# Variables for installation directories.
+# https://www.gnu.org/software/make/manual/html_node/Directory-Variables.html
+
+DESTDIR :=
+
+prefix := /usr/local
+exec_prefix := $(prefix)
+bindir := $(exec_prefix)/bin
+datarootdir := $(prefix)/share
+datadir := $(datarootdir)
+# libdir := $(prefix)/lib
+mandir := $(datarootdir)/man
+man1dir := $(mandir)/man1
+
+.PHONY: all
+all: ./agenda.1.gz
 
 .PHONY: install
 install: ./agenda.1.gz
-	install -m 755 -D ./agenda $(PREFIX)/bin/agenda
-	install -m 644 -D ./agenda.1.gz $(PREFIX)/share/man/man1/agenda.1.gz
-	install -m 644 -D ./completion.bash $(PREFIX)/share/bash-completion/completions/agenda
+	install -m 755 -D ./agenda $(DESTDIR)$(bindir)/agenda
+	install -m 644 -D ./agenda.1.gz $(DESTDIR)$(man1dir)/agenda.1.gz
+	install -m 644 -D ./completion.bash $(DESTDIR)$(datarootdir)/bash-completion/completions/agenda
 
 .PHONY: uninstall
 uninstall:
-	rm -f $(PREFIX)/bin/agenda
-	rm -f $(PREFIX)/share/man/man1/agenda.1.gz
-	rm -f $(PREFIX)/share/bash-completion/completions/agenda
+	rm -f $(DESTDIR)$(bindir)/agenda
+	rm -f $(DESTDIR)$(man1dir)/agenda.1.gz
+	rm -f $(DESTDIR)$(datarootdir)/bash-completion/completions/agenda
 
 .PHONY: clean
 clean:
 	rm -f ./agenda.1.gz
+
+./agenda.1.gz: ./agenda.1
+	gzip -fk ./agenda.1