Follows GNU Coding Standards, see
https://www.gnu.org/prep/standards/html_node/Standard-Targets.html#Standard-Targets

Signed-off-by: Matthias Loibl <mail@matthiasloibl.com>

These are to follow the GNU Coding Standards Makefile Targets:
https://www.gnu.org/prep/standards/html_node/Standard-Targets.html

This allows the user to specify build flags from the make command
line. For example to force a complete rebuild one could use:

make BUILD_FLAGS='-v -a' TAGS="sqlite"

- update required version of git-module for #2373

- User profile un/follow
- List user's followers/following

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.