This is achieved by adding public/css/gogs.css to special .IGNORE target, which
makes inability to generate/update gogs.css non-fatal and not stopping whole
build process. User is still notified about missing lessc command though, since
inability to update CSS may lead to potential problems:

    lessc public/less/gogs.less public/css/gogs.css
    make: lessc: No such file or directory
    make: [public/css/gogs.css] Error 1 (ignored)

More info at:

  https://www.gnu.org/software/make/manual/html_node/Special-Targets.html

This is pure cleanup commit.

This will ensure that running `make` we will get all necessary files built and
we do not need manually remember to rebuild them.

This speeds up single build/rebuild rather than install & build which compiles
everything twice, we just copy installed binary back to the project root.