Test Guile 2.0 and 2.2 during CI.

* build-aux/gitlab-ci.yml (before-script): Use a variable for the
Guile package version.
(test): Rename this...
(test-guile-2.2): ...to this and set the Guile version variable.
(test-guile-2.0): New job.

build-aux/gitlab-ci.yml

@@ -22,13 +22,21 @@ before_script:
   - apt-get update -qq && apt-get install -y -qq
       autoconf
       automake
-      guile-2.2-dev
+      guile-${TEST_GUILE_VERSION}-dev
       make
       pkg-config
       texinfo
   - which guile
   - guile --version
 
-test:
+test-guile-2.0:
+  variables:
+    TEST_GUILE_VERSION: "2.0"
+  script:
+    - ./bootstrap && ./configure && make && make check
+
+test-guile-2.2:
+  variables:
+    TEST_GUILE_VERSION: "2.2"
   script:
     - ./bootstrap && ./configure && make && make check