build: cmake fixes

- add EXTERNALPROJECT_OPTIONS variable to main build
- use `REQUIRED` keyword for IWYU.
- remove check_c_compiler_flag checks when `ENABLE_COMPILER_SUGGESTIONS`
  is `ON`. If we explicitly enable it then we probably want it to give
  an error if it doesn't exist, rather than silently skip it.
- Move dependency interface libraries to their find module and use them
  as a pseudo-imported target.
- Remove BUSTED_OUTPUT_TYPE. It's not used and we can reintroduce it
  again if something similar is needed.
- Use LINK_OPTIONS intead of LINK_FLAGS when generating the `--version`
  output.

cmake/FindLibintl.cmake

@@ -78,3 +78,9 @@ endif()
 find_package_handle_standard_args(Libintl DEFAULT_MSG
   ${REQUIRED_VARIABLES})
 mark_as_advanced(LIBINTL_LIBRARY LIBINTL_INCLUDE_DIR)
+
+add_library(libintl INTERFACE)
+target_include_directories(libintl SYSTEM BEFORE INTERFACE ${LIBINTL_INCLUDE_DIR})
+if (LIBINTL_LIBRARY)
+  target_link_libraries(libintl INTERFACE ${LIBINTL_LIBRARY})
+endif()