Minimum version requirement

[davecheney]

At the moment gb assumes go 1.4 as a minimum requirement, we do allow you to change the toolchain with the -goroot flag, but this was mainly for convenience.

When Go 1.5 ships, there will be a significant change to the toolchain, which I would prefer not to have to sniff at runtime.

Some options

• base the gc toolchain on the version of the toolchain that built gb using // +build go1.5 or something.
• raise the minimum requirement to Go 1.5

This also impacts the cross compilation story

[davecheney]

I've decide that gb will support go 1.4 and go 1.5 (and obviously additional versions in the future). To do this (and close this bug) the following steps need to be completed.

1. split gc.go into gc14.go and gc15.go to take into account the differences in compiler toolchains
2. remove -goroot
3. document that the gb install is tightly coupled to the version of Go that built it. This requirement is identical to the go tool, but it has the benefit of being built alongside the compiler and std lib, so the requirement is implicit. gb will have to make this an explicit requirement and building for different Go versions will require different versions of gb compiled with their respective toolchain
4. faq and documentation for the above.