Support environment variables

The way we work around SR-1636 is we add a C language target with atbuild's new C project syntax.

This C target can access `environ` and the result follows.

I think this bootstraps OK, we'll see what CI says

See also: https://bugs.swift.org/browse/SR-1636, https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160523/019350.html
2 jobs for master in 28 seconds (queued for 13 seconds)
Status Job ID Name Coverage
  Build
failed #6804
autoscale-linux
linux

00:00:20

failed #6805
openswift xcodeselect8
osx

00:00:07

 
Name Stage Failure
failed
linux Build There is an unknown failure, please try again
Submodule path 'atpkg/atfoundation': checked out '0f49f75f602539077f4bdeb4c5f37f999bab61f2'
$ bootstrap/build.sh linux
Compile Swift Module 'atfoundation' (17 sources)
Linking Library: .atllbuild/products/atfoundation.a
Compile Swift Module 'atpkg' (9 sources)
Linking Library: .atllbuild/products/atpkg.a
<unknown>:0: error: unable to open 'bootstrap/bootstrap-linux-atenvironment.swift-build'
<unknown>:0: error: unable to load build file
ERROR: Build failed: exit code 1
failed
osx Build There is an unknown failure, please try again
$ export PATH=/Library/Developer/Toolchains/swift-latest.xctoolchain/usr/bin:"${PATH}"
$ ./bootstrap/build.sh
Compile Swift Module 'atfoundation' (17 sources)
Linking Library: .atllbuild/products/atfoundation.a
Compile Swift Module 'atpkg' (9 sources)
Linking Library: .atllbuild/products/atpkg.a
<unknown>:0: error: unable to open 'bootstrap/bootstrap-macosx-atenvironment.swift-build'
<unknown>:0: error: unable to load build file
ERROR: Build failed: exit status 1