build: replace `string` with JOIN option to `string_join` helper in CMake

`string` with JOIN option is only available since CMake 3.12, but we have
developers using CMake 3.1: implement a utility `string_join` function to
remove this dependency.

Closes #5881

NO_CHANGELOG=<build fix>
NO_DOC=<build fix>
NO_TEST=<build fix>

(cherry picked from commit 55298308)