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 2 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #6810
autoscale-linux
linux

00:02:04

failed #6811
openswift xcodeselect8
osx

00:00:34

 
Name Stage Failure
failed
osx Build There is an unknown failure, please try again
         ^
1 error generated.
In file included from lib/baz.c:1:
lib/baz.h:6:10: fatal error: 'curl/curl.h' file not found
#include <curl/curl.h>
^
1 error generated.
<unknown>:0: error: build had 2 command failures
ERROR: Build failed: exit status 1