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 1585 minutes and 20 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
failed #6808
autoscale-linux
linux

26:25:20

failed #6809
openswift xcodeselect8
osx

00:00:19

 
Name Stage Failure
failed
linux Build There is an unknown failure, please try again
Running with gitlab-ci-multi-runner 1.7.0 (c66b00d)
Using Docker executor with image drewcrawford/buildbase:latest ...
Pulling docker image drewcrawford/buildbase:latest ...
failed
osx Build There is an unknown failure, please try again
Running task atbuild.atbuild with overlays [] for platform osx
Compile Swift Module 'atbuild' (1 sources)
Linking executable .atllbuild/products/atbuild
ld: warning: object file (.atllbuild/products/atenvironment.a(environment.c.o)) was built for newer OSX version (10.12) than being linked (10.9)
Completed task atbuild.atbuild.
Built package atbuild.
****************ENVIRONMENT VARIABLE TEST**************
./tests/test.sh: line 17: cd: /Users/chell-public/builds/cd08e98e/0/AnarchyTools/atbuild/tests/fixtures/environment: No such file or directory
ERROR: Build failed: exit status 1