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