[gitlab-ci] Test make maintainer-clean

Also install flex

git-svn-id: http://svn.osgeo.org/postgis/trunk@17433 b70326c6-7e19-0410-871a-916f4a2858ee

.gitlab-ci.yml

@@ -2,7 +2,7 @@ test:
   image: debian:buster
   script:
     - apt-get update -qq && apt-get install -y
-        bison
+        bison flex
         postgresql-11 postgresql-server-dev-11
         build-essential autoconf libtool libcunit1-dev
         xsltproc docbook-xsl docbook-mathml dblatex
@@ -12,6 +12,9 @@ test:
     - service postgresql start
     - su -l postgres -c "createuser -s `whoami`"
     - ./autogen.sh
+    - ./configure
+    - make maintainer-clean
+    - ./autogen.sh
     - ./configure CFLAGS="-Wall -fno-omit-frame-pointer -Werror"
     - make
     - make check RUNTESTFLAGS=-v