use "standard" posix directory variables

- dspinellis#64 (comment)
- https://www.gnu.org/software/make/manual/html_node/Directory-Variables.html
- https://www.gnu.org/software/automake/manual/html_node/Standard-Directory-Variables.html

Makefile

@@ -1,20 +1,20 @@
-PREFIX ?= /usr/local
-BINPREFIX ?= "$(PREFIX)/bin"
-LIBPREFIX ?= "$(PREFIX)/lib"
-MANPREFIX ?= "$(PREFIX)/share/man/man1"
-SYSCONFDIR ?= $(PREFIX)/etc
+prefix      := /usr/local
+exec_prefix := $(prefix)
+bindir      := $(exec_prefix)/bin
+libexecdir  := $(exec_prefix)/libexec
+sysconfdir  := $(prefix)/etc
+datarootdir := $(prefix)/share
+mandir      := $(datarootdir)/man
+man1dir     := $(mandir)/man1
 
 default: install
 
 install:
-	@mkdir -p $(DESTDIR)$(MANPREFIX)
-	@mkdir -p $(DESTDIR)$(BINPREFIX)
-	@mkdir -p $(DESTDIR)$(LIBPREFIX)/git-issue
-	install git-issue.sh $(DESTDIR)$(BINPREFIX)/git-issue
-	install lib/git-issue/import-export.sh $(DESTDIR)$(LIBPREFIX)/git-issue/import-export.sh
-	install -m 644 git-issue.1 $(DESTDIR)$(MANPREFIX)/
-	@mkdir -p $(DESTDIR)$(SYSCONFDIR)/bash_completion.d
-	install -m 644 gi-completion.sh $(DESTDIR)$(SYSCONFDIR)/bash_completion.d/git-issue
+	mkdir -p "$(bindir)" "$(libexecdir)/git-issue" "$(sysconfdir)/bash_completion.d" "$(man1dir)"
+	install git-issue.sh $(bindir)/git-issue
+	install lib/git-issue/import-export.sh $(libexecdir)/git-issue/import-export.sh
+	install -m 644 git-issue.1 $(man1dir)/
+	install -m 644 gi-completion.sh $(sysconfdir)/bash_completion.d/git-issue
 
 # Synchronize man page and usage with the contents of the README file
 sync-docs:
@@ -29,9 +29,9 @@ test:
 	./test.sh
 
 uninstall:
-	rm -f $(DESTDIR)$(BINPREFIX)/git-issue
-	rm -f $(DESTDIR)$(MANPREFIX)/git-issue.
-	rm -f $(DESTDIR)$(SYSCONFDIR)/bash_completion.d/git-issue
+	rm -f $(bindir)/git-issue
+	rm -f $(man1dir)/git-issue.
+	rm -f $(sysconfdir)/bash_completion.d/git-issue
 
 clean:
 

git-issue.sh

@@ -36,7 +36,7 @@ IFS=:
 # Set library path
 # shellcheck disable=SC2086
 # Rationale: Word splitting not an issue
-LIB_PATH="$(dirname $0)/../lib:$LD_LIBRARY_PATH:/usr/local/lib:/usr/lib"
+LIB_PATH="$(dirname $0)/../libexec:$LD_LIBRARY_PATH:/usr/local/libexec:/usr/libexec"
 if [ "x$GIT_ISSUE_LIB_PATH" != x ] ; then
   LIB_PATH="$GIT_ISSUE_LIB_PATH"
 fi