misc/dist: add -version flag to override version name
misc/dist: add -version flag to override version name

Also, don't build the tour when making the source distribution.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/8354043

apparentlymart/golang,dsriram/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,dsriram/golang,dsriram/golang,gnanderson/go,tv42/old-go,gnanderson/go,apparentlymart/golang,dsriram/golang,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,tv42/old-go,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,tv42/old-go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang