configure: Add --with-pkg{data,sysconf,libexec}dir options

Quick hack for discussion.

Makefile.in

@@ -22,11 +22,11 @@ libdir	= @libdir@
 incdir	= @includedir@
 mandir	= @mandir@
 datadir = @datadir@
-pkgdatadir = @datadir@/@PACKAGE_NAME@
+pkgdatadir = @pkgdatadir@
 sysconfdir = @sysconfdir@
-pkgsysconfdir = @sysconfdir@/@PACKAGE_NAME@
+pkgsysconfdir = @pkgsysconfdir@
 libexecdir = @libexecdir@
-pkglibexecdir = @libexecdir@/@PACKAGE_NAME@
+pkglibexecdir = @pkglibexecdir@
 SLINK	= @LN_S@
 STRIP	= @STRIP@
 ifdef SPARSE

configure.ac

@@ -200,11 +200,47 @@ AC_ARG_ENABLE(tmpdir,
 		[default directory for temporary files [ARG=/tmp]])],
 	tmpdir_specified=yes)
 
+AC_ARG_WITH([pkgdatadir],
+	[AS_HELP_STRING([--with-pkgdatadir=DIR],
+		[ctags read-only architecture-independent data [DATADIR/ctags]])])
+
+AC_ARG_WITH([pkgsysconfdir],
+	[AS_HELP_STRING([--with-pkgsysconfdir=DIR],
+		[ctags read-only single-machine data [SYSCONFDIR/ctags]])])
+
+AC_ARG_WITH([pkglibexecdir],
+	[AS_HELP_STRING([--with-pkglibexecdir=DIR],
+		[ctags program executables [LIBEXECDIR/ctags]])])
+
 AC_ARG_PROGRAM
 
 # Process configuration options
 # -----------------------------
 
+
+case "${with_pkgdatadir+set}"-"$with_pkgdatadir" in #(
+"set"-"yes" | "set"-"no") pkgdatadir="${datadir}/ctags"	  ;; #(
+"set"-*)		  pkgdatadir="${with_pkgdatadir}" ;; #(
+*)			  pkgdatadir="${datadir}/ctags"	  ;;
+esac
+
+case "${with_pkgsysconfdir+set}"-"$with_pkgsysconfdir" in #(
+"set"-"yes" | "set"-"no") pkgsysconfdir="${sysconfdir}/ctags"	;; #(
+"set"-*)		  pkgsysconfdir="${with_pkgsysconfdir}" ;; #(
+*)			  pkgsysconfdir="${sysconfdir}/ctags"	;;
+esac
+
+case "${with_pkglibexecdir+set}"-"$with_pkglibexecdir" in #(
+"set"-"yes" | "set"-"no") pkglibexecdir="${libexecdir}/ctags"	;; #(
+"set"-*)		  pkglibexecdir="${with_pkglibexecdir}" ;; #(
+*)			  pkglibexecdir="${libexecdir}/ctags"	;;
+esac
+
+AC_SUBST([pkgdatadir])
+AC_SUBST([pkgsysconfdir])
+AC_SUBST([pkglibexecdir])
+
+
 install_targets="install-ctags install-data install-libexec"
 AC_MSG_CHECKING(whether to install link to etags)
 if test yes = "$enable_etags"; then