Fix compile command to handle correctly the optional argument <client>

Fix compile command to handle correctly the optional argument <client>
In case the first arg (eventually after -c|-n):
- it's an empty string "" just skip it (default clients will be used).
- doesn't contain any valid client, then assume it's not a client option but a generic option.
- contains at least one valid client, then compile valid ones and warn about invalid ones.

Now finally
./all compile -n FTEQCCFLAGS_WATERMARK=
works :)