若gbb.json的tool中指定了-ldflags选项，最终调用gb/go build/go install时，需要合并-ldflags选项的值

[voidint]

go build -ldflags '-w' -ldflags '-X "github.com/voidint/gbb/build.Date=2017-04-25T16:37:18+08:00" -X "github.com/voidint/gbb/build.Commit=9b1921adb355700fd522c1c24a1f1e2a9279e3f5"'

转化为

go build -ldflags '-X "github.com/voidint/gbb/build.Date=2017-04-25T16:37:18+08:00" -X "github.com/voidint/gbb/build.Commit=9b1921adb355700fd522c1c24a1f1e2a9279e3f5" -w'

[voidint]

若指定了多个-ldflags选项，则最后一个有效。