Turn C++ compiler warning into error on the right job. Re #85

Fixes 8d67531c70 where the flags were
set for a job that runs in an environment *without* a C++ compiler.

.gitlab-ci.yml

@@ -49,14 +49,14 @@ compile:debian-9-mini:
   image: $REGISTRY_HUB:debian-9-mini
   variables:
     CONFIGURE_OPTS: "$CONFIGURE_MINI"
-    MAKE_FLAGS: "CFLAGS=-Werror CXXFLAGS=-Werror"
+    MAKE_FLAGS: "CFLAGS=-Werror"
   <<: *compile_definition
 
 compile:debian-9-full:
   image: $REGISTRY_HUB:debian-9-full
   variables:
     CONFIGURE_OPTS: "$CONFIGURE_MINI $CONFIGURE_FULL"
-    MAKE_FLAGS: "CFLAGS=-Werror"
+    MAKE_FLAGS: "CFLAGS=-Werror CXXFLAGS=-Werror"
   <<: *compile_definition
   after_script:
   - make -C doc html-pages