Issue Description:

On macOS, swift build hardcodes the swiftc flag

-target x86_64-apple-macosx10.10

This means that to write portable (macOS 10.11+ and Linux) code that uses a Foundation API introduced in macOS 10.11 we must convert the following straightforward code (2 module-level URL constants):

import Foundation

let baseURL: URL = ...
let resourceURL = URL(fileURLWithPath: "path", relativeTo: baseURL)

to this

import Foundation

let baseURL: URL = ...
let resourceURL: URL
#if !os(Linux)
    resourceURL = {
        guard #available(macOS 10.11, *) else {
            fatalError("This program requires macOS 10.11 or greater.")
        }
        return URL(fileURLWithPath: "path", relativeTo: baseURL)
    }()
#else
    resourceURL = URL(fileURLWithPath: "path", relativeTo: baseURL)
#endif

Swift packages should be able to specify a minimum target platform version. There's also the problem of the impedance mismatch - if we were allowed to do #if os(Linux) && available(macOS 10.11, *) the resourceURL assignment line would not need to be duplicated in each block.