gitlab: add explicit early job for syntax-check

Running the code style syntax-check as part of the build jobs leads to
all jobs failing in the same way. Have a prebuild job for validating
syntax-check to catch code style problems upfront and thus avoid needing
to run all the build jobs.
Reviewed-by: NErik Skultety <eskultet@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>a
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

@@ -181,6 +181,16 @@ website:
       - website
 
 
+codestyle:
+  stage: prebuild
+  script:
+    - mkdir build
+    - cd build
+    - ../autogen.sh || (cat config.log && exit 1)
+    - $MAKE -j $(getconf _NPROCESSORS_ONLN) syntax-check
+  image: quay.io/libvirt/buildenv-libvirt-fedora-31:latest
+
+
 # This artifact published by this job is downloaded to push to Weblate
 # for translation usage:
 #    https://gitlab.com/libvirt/libvirt/-/jobs/artifacts/master/download?job=potfile