Test the pre-generated man pages in GitLab CI

Add an extra job for a build from tarball, but without sphinx-build and
enable RUN_MAKE_INSTALL to check that man pages were generated and
installed.

Disable the RUN_MAKE_INSTALL on the systems without sphinx-build (sid).

(cherry picked from commit 8f2e1e15cc)

.gitlab-ci.yml

@@ -670,6 +670,21 @@ unit:gcc:oraclelinux9:amd64:
     - job: gcc:oraclelinux9:amd64
       artifacts: true
 
+gcc:tarball:nosphinx:
+  variables:
+    CC: gcc
+    CFLAGS: "${CFLAGS_COMMON}"
+    EXTRA_CONFIGURE: "--with-libidn2 --disable-developer"
+    RUN_MAKE_INSTALL: 1
+  <<: *oraclelinux_9_amd64_image
+  <<: *build_job
+  before_script:
+    - (! command -v sphinx-build >/dev/null)
+    - *unpack_release_tarball
+  needs:
+    - job: tarball-create
+      artifacts: true
+
 # Jobs for regular GCC builds on Debian 10 "buster" (amd64)
 
 gcc:buster:amd64: