build: Let 'make' generate 'src/cuirass/config.scm'.

* Makefile.am (src/cuirass/config.scm, $(go_files)): New targets.
(do_subst, generate_file, go_files): New variables.
(EXTRA_DIST): Distribute 'src/cuirass/config.scm.in'.
(DISTCLEANFILES): Rename to ...
(MOSTLYCLEANFILES): ... this.
* configure.ac (AC_CONFIG_FILES): Remove 'src/cuirass/config.scm'.
(@expanded_datadir@, @expanded_localstatedir@): Delete.
(AC_PROG_MKDIR_P, AC_PROG_SED): Use them.
* src/cuirass/config.scm (%datadir): Set it to $(localstatedir).
(%localstatdir): Set it to $(datadir).

Makefile.am

@@ -86,18 +86,60 @@ sql-check: src/schema.sql
 EXTRA_DIST = \
   .dir-locals.el \
   bootstrap \
+  src/cuirass/config.scm.in \
   tests/gnu-system.scm \
   tests/guix-jobs.scm \
   tests/hello-singleton.scm \
   tests/hello-subset.scm \
   $(TESTS)
 
-DISTCLEANFILES = src/cuirass/config.scm
+MOSTLYCLEANFILES = src/cuirass/config.scm
 CLEANFILES = \
   $(nodist_guilesite_DATA) \
   $(dist_pkgmodule_DATA:%.scm=%.go) \
   src/cuirass/config.go
 
+## ----------------- ##
+## Generated files.  ##
+## ----------------- ##
+
+# 'AC_CONFIG_FILES' doesn't fully expand variables that depend on ${prefix}.
+# To use such variables in source files and scripts while following GNU Coding
+# Standards, let 'make' manage their replacement.
+
+# Use config.status to substitute the remainder where a single expansion is
+# sufficient.  We use a funny notation here to avoid configure substitutions
+# in our text.
+do_subst = ( $(SED) \
+  -e "s,@configure_input[@],Generated from $$in.  Do not edit by hand.,g" \
+  -e 's,@datadir[@],$(datadir),g' \
+  -e 's,@localstatedir[@],$(localstatedir),g' \
+  | $(SHELL) ./config.status --file=- )
+
+# Generic instructions to perform the substitution.  Generated files shouldn't
+# contain unexpanded '@substitutions@', and should be made read-only, to
+# prevent them from being edited by mistake instead of the file the are
+# generated from.
+generate_file = \
+  $(AM_V_GEN)rm -f $@ $@-t; \
+  $(MKDIR_P) $(@D); \
+  in=$@.in && $(do_subst) <$(srcdir)/$$in >$@-t; \
+  if LC_ALL=C grep '@[a-zA-Z0-9_][a-zA-Z0-9_]*@' $@-t; then \
+    echo "$@ contains unexpanded substitution (see lines above)"; \
+    exit 1; \
+  fi; \
+  chmod a-w $@-t; mv -f $@-t $@
+
+# This file depends on Makefile so it is rebuilt if $(VERSION),
+# $(datadir) or other do_subst'ituted variables change.
+src/cuirass/config.scm: $(srcdir)/src/cuirass/config.scm.in Makefile
+	$(generate_file)
+
+# Guile modules require 'src/cuirass/config.scm' to exist before being
+# compiled.
+go_files = $(nodist_guilesite_DATA) $(dist_pkgmodule_DATA:%.scm=%.go)
+$(go_files): src/cuirass/config.scm
+
 ## -------------- ##
 ## Silent rules.  ##
 ## -------------- ##

configure.ac

@@ -8,14 +8,8 @@ AM_INIT_AUTOMAKE([foreign subdir-objects -Wall])
 AM_SILENT_RULES([yes])		# enables silent rules by default
 
 AC_CANONICAL_HOST
-
-# Prepare a version of installation directories that does not contain
-# references to shell variables.
-expanded_prefix="`eval echo $prefix | sed -e "s|NONE|/usr/local|g"`"
-AC_SUBST([expanded_datadir],
-  [`eval echo $datarootdir | sed -e "s|NONE|$expanded_prefix|g"`])
-AC_SUBST([expanded_localstatedir],
-  [`eval echo $localstatedir | sed -e "s|NONE|$expanded_prefix|g"`])
+AC_PROG_MKDIR_P
+AC_PROG_SED
 
 PKG_CHECK_MODULES([GUILE], [guile-2.0 >= 2.0.7])
 AC_PATH_PROG([GUILE], [guile])
@@ -27,8 +21,7 @@ GUILE_MODULE_REQUIRED([guix])
 GUILE_MODULE_REQUIRED([json])
 GUILE_MODULE_REQUIRED([sqlite3])
 
-AC_CONFIG_FILES([Makefile
-                 src/cuirass/config.scm])
+AC_CONFIG_FILES([Makefile])
 AC_CONFIG_FILES([bin/cuirass], [chmod +x bin/cuirass])
 AC_CONFIG_FILES([bin/evaluate], [chmod +x bin/evaluate])
 AC_CONFIG_FILES([pre-inst-env:build-aux/pre-inst-env.in],

src/cuirass/config.scm.in

-;;; config.scm -- variables defined at configure time
+;;;; config.scm -- constants defined by the build system.
+;;; @configure_input@
 ;;; Copyright © 2016 Mathieu Lirzin <mthl@gnu.org>
 ;;;
 ;;; This file is part of Cuirass.
@@ -48,8 +49,8 @@
 
 (define-public %datadir
   ;; Define to DATADIR without reference to '${prefix}'.
-  "@expanded_datadir@")
+  "@datadir@")
 
 (define-public %localstatedir
   ;; Define to LOCALSTATEDIR without reference to '${prefix}'.
-  "@expanded_localstatedir@")
+  "@localstatedir@")