BlueSSLService cannot be cross-compiled

[weissi]

BlueSSLService uses #if os(...) in the Package.swift file which prevents it from being cross-compiled because that makes the assumption that the Package.swift is executed on the same OS as the resulting binaries will be which isn't true for cross-compilation.

I know this is to make it compatible with macOS without installing Open/LibreSSL using homebrew on macOS and I don't know if there's a full solution to get it to properly work with SwiftPM. There might be a way using conditional imports in the .swift files rather than in the Package.swift but I'm not 100% sure.

The other option is that we do something hacky and we change the

#if os(Linux)

to something like

#if CROSS_COMPILATION_TARGET_LINUX || os(Linux)

and in the cross compilation toolchain builder script we'd just add

-DCROSS_COMPILATION_TARGET_LINUX

to the extra-swiftc-flags list.

[weissi]

I prepared a PR for that: swiftlang/swift-package-manager#2114