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

Test with a Clang version old enough to have caught the regression fixed
by commit 9411766c (BinUtils: Avoid llvm-objdump versions older than
Clang 9, 2021-05-29).

.gitlab-ci.yml

@@ -176,6 +176,16 @@ test:debian10-aarch64-ninja:
     variables:
         CMAKE_CI_NO_MR: "true"
 
+test:debian10-makefiles-clang:
+    extends:
+        - .debian10_makefiles_clang
+        - .cmake_test_linux_release
+        - .linux_builder_tags_qt
+        - .run_manually
+        - .needs_centos6_x86_64
+    variables:
+        CMAKE_CI_JOB_NIGHTLY: "true"
+
 test:fedora34-makefiles:
     extends:
         - .fedora34_makefiles

.gitlab/ci/configure_debian10_makefiles_clang.cmake

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

.gitlab/ci/env_debian10_makefiles_clang.sh

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

.gitlab/os-linux.yml

@@ -149,6 +149,15 @@
         CTEST_NO_WARNINGS_ALLOWED: 1
         CMAKE_GENERATOR: "Unix Makefiles"
 
+### Clang Compiler
+
+.debian10_makefiles_clang:
+    extends: .debian10
+
+    variables:
+        CMAKE_CONFIGURATION: debian10_makefiles_clang
+        CMAKE_GENERATOR: "Unix Makefiles"
+
 ### Intel Compiler
 
 .intelcompiler: