👷 Add clang-tidy canary

Problem:
- `clang-tidy` exits with success if the `.clang-tidy` file is ill-formed.

Solution:
- Add a clang-tidy-canary target which tests if `clang-tidy` has any error
  output when run with `--verify-config`.

Author: elbeno

test/library/CMakeLists.txt

@@ -32,4 +32,16 @@ target_sources(
 if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
     add_docs(docs)
     clang_tidy_interface(TARGET test_lib EXCLUDE_FILESETS exclusions)
+
+    if(CLANG_TIDY_PROGRAM)
+        add_custom_command(
+            OUTPUT clang_tidy_canary.alive
+            COMMAND ${CLANG_TIDY_PROGRAM} "--verify-config" 2>clang_tidy.log
+            COMMAND "!" "[" "-s" "clang_tidy.log" "]"
+            COMMAND ${CMAKE_COMMAND} "-E" "touch" "clang_tidy_canary.alive"
+            DEPENDS ${CMAKE_SOURCE_DIR}/.clang-tidy)
+        add_custom_target(clang-tidy-canary DEPENDS clang_tidy_canary.alive)
+        add_dependencies(quality clang-tidy-canary)
+        add_dependencies(ci-quality clang-tidy-canary)
+    endif()
 endif()