CI: Do not use silent rules

Instead use make(1) env MAKEFLAGS set to `-w -O`.

With silent rules we don't see compile commands and only see a error
messages. This is beautiful when there is no errors but harder to
understand the cause when there are errors.

For example, after adding `-Wno-error=unused-but-set-variable`
I still see the error:

  CXX      genesys/libgenesys_la-device.lo
genesys/genesys.cpp:843:19: error: variable 'i' set but not used [-Werror,-Wunused-but-set-variable]
  843 |     for (unsigned i = 0;; ++i) {
      |                   ^

But why? Hard to tell since we don't see compile command line.

With `-w -O` we see each compilation job grouped in the log.

This will help to debug build issues in CI, where reproduction is
costly/harder for a casual contributor. Compilation command is just
above the error. Find errors in log by searching for `error:` or
`warning:` strings.

.gitlab-ci.yml

@@ -5,7 +5,7 @@
 
 variables:
   REGISTRY_HUB: "registry.gitlab.com/sane-project/ci-envs"
-  CONFIGURE_MINI: "--enable-silent-rules"
+  CONFIGURE_MINI: ""
   CONFIGURE_FULL: "--with-usb --with-usb-record-replay --with-avahi --enable-pnm-backend --with-libcurl --with-poppler-glib"
 
 stages:
@@ -39,7 +39,7 @@ make-dist:
   - cd build
   - tar xzf ../sane-backends-*.tar.gz --strip-components=1
   - (set -x; ./configure $CONFIGURE_OPTS)
-  - eval "(set -x; make -j2 -k $MAKE_FLAGS)"
+  - eval "(set -x; make -w -O -j2 -k $MAKE_FLAGS)"
 
 debian-11-mini:
   image: $REGISTRY_HUB:debian-bullseye-mini