build: Prefer building with Guile 2.2.

* Makefile.am (guilesitedir): Append @GUILE_EFFECTIVE_VERSION@.
* configure.ac: Use 'GUILE_PKG', and pass "2.2" first.

Makefile.am

@@ -21,7 +21,7 @@
 bin_SCRIPTS = bin/cuirass bin/evaluate
 noinst_SCRIPTS = pre-inst-env
 
-guilesitedir = $(datarootdir)/guile/site/2.0
+guilesitedir = $(datarootdir)/guile/site/@GUILE_EFFECTIVE_VERSION@
 dist_guilesite_DATA = src/cuirass.scm
 nodist_guilesite_DATA = $(dist_guilesite_DATA:.scm=.go)
 

configure.ac

 ## Process this file with autoconf to produce a configure script.
 
 # Copyright © 2016, 2017 Mathieu Lirzin <mthl@gnu.org>
+# Copyright © 2017 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of Cuirass.
 #
@@ -33,7 +34,12 @@ AC_CANONICAL_HOST
 AC_PROG_MKDIR_P
 AC_PROG_SED
 
-PKG_CHECK_MODULES([GUILE], [guile-2.0 >= 2.0.7])
+GUILE_PKG([2.2 2.0])
+
+if test "x$GUILE_EFFECTIVE_VERSION" = "x2.0"; then
+  PKG_CHECK_MODULES([GUILE], [guile-2.0 >= 2.0.7])
+fi
+
 AC_PATH_PROG([GUILE], [guile])
 AC_PATH_PROG([GUILD], [guild])
 AS_IF([test -z "$ac_cv_path_GUILD"],