build: Allow builds with Guile 3.0.

* configure.ac: Add "3.0" to 'GUILE_PKG'.
* README: Mention it.

README

@@ -6,7 +6,7 @@ Requirements
 
 Cuirass currently depends on the following packages:
 
-  - GNU Guile 2.2.x or later
+  - GNU Guile 3.0.x or 2.2.x
   - GNU Guix (and all its development dependencies)
   - GNU Make
   - Guile-JSON 3.x

configure.ac

 ## Process this file with autoconf to produce a configure script.
 
 # Copyright © 2016, 2017 Mathieu Lirzin <mthl@gnu.org>
-# Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2017, 2018, 2020 Ludovic Courtès <ludo@gnu.org>
 # Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 #
 # This file is part of Cuirass.
@@ -36,7 +36,7 @@ AC_PROG_MKDIR_P
 AC_PROG_SED
 
 # We need Fibers, which requires 2.2+.
-GUILE_PKG([2.2])
+GUILE_PKG([3.0 2.2])
 
 AC_PATH_PROG([GUILE], [guile])
 AC_PATH_PROG([GUILD], [guild])