[Docs] Added information about building toolchains

[wokalski]

It took me a long time to find this information and I feel like it has to be better documented. If it is not an appropriate place, let me know.

[mxcl]

We don't really expect people to build their own, we release them as often as possible. Why did you feel the need to build your own?

[ketzusaka]

Would building your own help a contributor make contributions?

[aciidgh]

Building toolchain has been helpful recently due to breaking change in XCTests and the swift-3-api-guidelines merge but I think this should be in swift repo docs and not here

[wokalski]

@mxcl excellent question. I was pondering in SPM and tried to build my package with a local version of SPM but got an error that my swiftc is not compatible (I installed the latest development snapshot).

I also built Swift locally but had no luck using it. I could symlink to it from /usr/bin, but I'd rather have this structured workflow where I build a local toolchain and have it there, and can always switch from/to it.

Maybe I did something wrong, I would be glad to document "the correct process" if you can point me in the right direction. Obviously somewhere where it fits.

@aciidb0mb3r Well, I thought so, intuitively. However the only place which mentions managing toolchains is this one. AFAIK, there's no documentation for build-script in the main repo either. This is why I decided to add it here.

[mxcl]

Building your own is not necessary here. The problem is that swift build must be run with the version of swift it is built with and probably it was using the version built with Xcode.

You can set SWIFT_EXEC to work around this.

Probably we should document that part. Building the toolchain is not something to be documented here.

[wokalski]

@mxcl Thanks! Setting SWIFT_EXEC fixed this. It was exactly what I hoped would exist but couldn't find.

I can either create a new document in Documentation/Internals or add a note in the README in Documentation section but long term it will get messy.

I can create a document called EnvironmentVariables.md and document it there. Does it sound good? Are there any environment variables you'd like to be documented?

[mxcl]

I think the README can mention this. It may get a bit messy, but we should clean it up at some point soon.