gitlab-ci: add job to test with clang-7 and ninja on debian10

In commit 7e5e558d (gitlab-ci: add job to test with clang-7 on
debian10, 2021-06-04) we added coverage of clang-7 with the Unix
Makefiles generator.  Do the same for Ninja.

.gitlab-ci.yml

@@ -176,6 +176,16 @@ test:debian10-aarch64-ninja:
     variables:
         CMAKE_CI_NO_MR: "true"
 
+test:debian10-ninja-clang:
+    extends:
+        - .debian10_ninja_clang
+        - .cmake_test_linux_release
+        - .linux_builder_tags_qt
+        - .run_manually
+        - .needs_centos6_x86_64
+    variables:
+        CMAKE_CI_JOB_NIGHTLY: "true"
+
 test:debian10-makefiles-clang:
     extends:
         - .debian10_makefiles_clang

.gitlab/ci/configure_debian10_ninja_clang.cmake

+include("${CMAKE_CURRENT_LIST_DIR}/configure_external_test.cmake")

.gitlab/ci/env_debian10_ninja_clang.sh

+export CC=/usr/bin/clang-7
+export CXX=/usr/bin/clang++-7

.gitlab/os-linux.yml

@@ -158,6 +158,12 @@
         CMAKE_CONFIGURATION: debian10_makefiles_clang
         CMAKE_GENERATOR: "Unix Makefiles"
 
+.debian10_ninja_clang:
+    extends: .debian10
+
+    variables:
+        CMAKE_CONFIGURATION: debian10_ninja_clang
+
 ### Sanitizers
 
 .fedora_memcheck: