Move gimp-debug-tools.c from tools/ to a new app-tools/ subdir,
which should contain external tools needed by app/, and which is
built *after* app/ (unlinke tools/). This allows gimp-debug-tool
to depend on libapp and libappwidgets, instead of on actual source
files from app/. Building sources from app/ in another subdir
screws with the distclean rules, and breaks distcheck.
