build: repair the build on 14.04

[compnerd]

CMake 3.4.3 does not support the ... pattern for
copy_if_different. Adjust for that.

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test macOS platform

[compnerd]

@swift-ci please test

[gottesmm]

@swift-ci test linux platform

[spevans]

@swift-ci test

[compnerd]

@swift-ci please test

[spevans]

Looking at the Ubuntu14.04 build host logs they show the cmake version as 3.4.3 but https://github.com/apple/swift/blob/master/docs/Ubuntu14.md says that 3.5.2 should be installed.

Maybe we should just get the 14.04/16.04 hosts updated to 3.10 and then set a minimum version check in Swift's CMakeLists.txt. Now that we are using CMake everywhere having to use such a low version may cause problems in the future.

[gottesmm]

@swift-ci please test

[gottesmm]

@spevans A few things:

1. We can not just update the minimum accepted cmake version across all projects. This is because changing the minimum required cmake for LLVM/clang is a non-trivial thing that takes community input. IIRC from talking with @llvm-beanz there isn't much of a push in LLVM to upgrade to a newer cmake yet. But I might be wrong. @llvm-beanz has this changed?

2. If we can't change the minimum version accepted by LLVM/clang, I would be hesitant to upgrade the minimum version for swift and all downstream dependencies since we use cmake chaining to pass information in between different cmake invocations. I don't think it is likely, but you could have a situation where the export format is different in between cmakes that are compatible with 3.4.3 and those that have later compatibility.

[spevans]

I didn't realise there would be those issues. But could we at least get the Ubuntu14 CI hosts installed with 3.5.2 as listed in the docs? - I have it on my Ubuntu14 test host and the full toolchain builds and tests ok without the need for the fix in this PR.

[gottesmm]

@spevans two things:

1. I do think it could be reasonable to upgrade swift's as long as we put something in build-script to check this. Otherwise if we upgraded swift's you wouldn't get a failure until after llvm/clang had been built (which would be annoying).

2. If we do not do such a thing, I would be worried about upgrading ubuntu14 CI unless we had some other bot to catch for incompatibilities (i.e. one bot running with a 3.4.3 cmake).

Doing 1 would require a post to swift-dev and potential discussion I think.

[gottesmm]

Btw, it looks like the macOS changes are related to some clang changes: https://ci.swift.org/job/oss-swift-incremental-RA-osx/5513/ and not to this PR. I am going to merge this to unbreak the 14.04 build.

[spevans]

@gottesmm This PR #1750 fixes the macOS build (and also building sclf when built in Xcode) but breaks linux - although I don't understand why tbh.