Allows linting only modified files and linting multiple files in
parallel. In the current state is rather slow because errors.json is
a 6 MiB file and needs to be reparsed each time.
Results on my system (6-core):
# In build dir, actually parallel
make -j5 clint 241.24s user 8.39s system 334% cpu 1:14.74 total
# In root, one process
make -j5 clint 60.69s user 0.37s system 93% cpu 1:05.19 total
In both cases download time included.
That is not well for travis (though I would keep travis as-is because
new variant will fail before checking all files), but already good
enough for regular development: total times are nearly identical and
this is the *full* build, further `make -C build clint` will check only
modified files.
