Misc improvements to the Gitlab-CI pipeline for MariaDB

- Add new Ninja and Clang build jobs. This helps to ensure those
  toolchains also work in addition to default CMake/gcc.

- Generate dependencies.dot/png to illustrate the CMake/Make/Ninja
  build dependencies. Viewing this image and identifying bottle necks
  in parallelism can help make the build run faster.

- Enable CUnit tests now as they are fixed on 10.6 (MDEV-25820).

- Limit parallel builds to 2 CPUs (full parallelism needs MDEV-25968) on
  CMake/Make. Now only the Ninja builds run full parallel builds as only
  Ninja is smart enough to prevent builds failing on resource
  over-consumption.

- Enable Gitlab-CI cache for job 'centos8' for ccache so that it builds
  faster. Don't use Gitlab-CI cache for other jobs, as it would too easily
  use up all free tier storage on Gitlab.com and force users to get a paid
  account just for MariaDB builds.

- On other jobs clean away ccache, as it only had a 5% hit rate on single
  builds with no downloaded cache.

- Dump full database contents during the test install so that one can
  use diff to compare the database contents at different stages and thus
  track/debug potential bugs in mariadb-install-db and mariadb-upgrade
  code.

Bugfixes:

- Zero out ccache stats before each run so that 'ccache -s' would actually
  show the stats for the latest run.

cmake/cpack_rpm.cmake

 IF(RPM)
 
+MESSAGE(STATUS "CPackRPM building with RPM configuration: ${RPM}")
+
 SET(CPACK_GENERATOR "RPM")
 SET(CPACK_RPM_PACKAGE_DEBUG 1)
 SET(CPACK_PACKAGING_INSTALL_PREFIX ${CMAKE_INSTALL_PREFIX})
@@ -274,7 +276,7 @@ FILE(GLOB compat101 RELATIVE ${CMAKE_SOURCE_DIR}
     "${CMAKE_SOURCE_DIR}/../MariaDB-shared-10.1.*.rpm")
 IF(compat53 AND compat101)
   FOREACH(compat_rpm "${compat53}" "${compat101}")
-    MESSAGE("Using ${compat_rpm} to build MariaDB-compat")
+    MESSAGE(STATUS "Using ${compat_rpm} to build MariaDB-compat")
     INSTALL(CODE "EXECUTE_PROCESS(
                    COMMAND rpm2cpio ${CMAKE_SOURCE_DIR}/${compat_rpm}
                    COMMAND cpio --extract --make-directories */libmysqlclient*.so.* -