This is for future changes with Meson, which doesn't allow a
different name for its `subprojects` directory. Having both a
`submodules` and a `subprojects` directory is confusing.
* .gitmodules, autogen.sh (copy_submodule_files, DLG_INC_DIR,
DLG_SRC_DIR): Updated.
* builds/toplevel.mk (<top-level>, do-dist),
builds/windows/vc2010/script.bat: Updated.
* src/tools/no-copyright: Updated.
