[POSIX] Inherit TOOLCHAINS when building from Xcode as well.
[POSIX] Inherit TOOLCHAINS when building from Xcode as well.

apple/swift-package-manager,JGiola/swift-package-manager,sschiau/swift-package-manager,JGiola/swift-package-manager,mxcl/swift-package-manager,mxcl/swift-package-manager,abertelrud/swift-package-manager,haskellswift/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,vmanot/swift-package-manager,mxcl/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,haskellswift/swift-package-manager,apple/swift-package-manager,vmanot/swift-package-manager,JGiola/swift-package-manager,haskellswift/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,mxcl/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,haskellswift/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,JGiola/swift-package-manager