Add all, check and dist Makefile rules (#3549)

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

Makefile

@@ -17,6 +17,12 @@ GOVET = go tool vet -composites=false -methods=false -structtags=false
 
 .IGNORE: public/css/gogs.css
 
+all: build
+
+check: test
+
+dist: release
+
 govet:
 	$(GOVET) gogs.go
 	$(GOVET) models modules routers