make --depends generate no CSS output

[gtalusan]

I'm using lessc with Qt/C++ and up until recently I've just been regenerating the stylesheets, but I thought that this could be faster.

I dug around and found the depends flag and it sped up my build process 0 to 💯 real quick.

The only downside is that when lessc is invoked twice, it will generate output twice. I actually only need the dependencies with no output, and then at a later stage during make, the actual output. I see here (#2476) that it's a nice to have feature, so here it is.

The new option is -Mn for short or --depends-no-output. It will exercise the same code path as depends but skip the output stage if depends_no_output is true.

[seven-phases-max]

I suspect not generating the output file with just the --depends option would look more consistent. (I don't use this feature so I may be missing a use-case where one could need both simultaneously, but #2476 (comment) suggest the same).

[gtalusan]

Okay, updated --depends to not generate output.

[seven-phases-max]

Thanks for the update. (I'm actually still in doubt if this can have some undesired side-effects, but since #2476 suggest the same, and it seems like nobody is going to comment this anyway (it's been a month the PR is hanging) - let's just merge and wait for issue reports when it's released :P).

[seven-phases-max]

Thanks!