New variable for Go packages, GOFLAGS

Hi!

A number of Go packages put stuff into GO_BUILD_PATTERN that's not 
strictly a Go build pattern. Often, these are "-ldflags" to set build 
dates and such. Several people have suggested that there should be a
separate setting for this.

I just committed a change to go-module.mk that adds a GOFLAGS variable for 
exactly this purpose. GO_BUILD_PATTERN should only contain package 
patterns, such as "./...", in the future.

I intend update all the existing Go packages in pkgsrc and wip soon.

-- 
Benny