[WIP] superenv: set effective_sysroot on 10.14 CLT

[mistydemeo]

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I'm mostly opening this for documentation. It's probably too early in the cycle to start committing workarounds like this.

The current version of the CLT doesn't include headers in the traditional Unixy locations, which breaks some software and which means that we end up not setting some special include paths. I've filed this bug as rdar://40833474, but meanwhile this workaround fixes broken builds like imagemagick.

If the traditional Unix locations aren't coming back, we'll have to look at wider-reaching changes, but I'll want to let more time pass before looking at something like that.

See also Homebrew/homebrew-core#28703 and Homebrew/homebrew-core#28733.

[andrewpile]

FWIW, the XCode 10 release notes make mention of the missing headers and provide a package for installing them in /usr/include, this is an additional step after installing CLT.

The Command Line Tools package installs the macOS system headers inside the macOS SDK.
Software that compiles with the installed tools will search for macOS headers at the standard
include path:

/Applications/Xcode-beta.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include

For legacy software that looks for the macOS headers in the base system under /usr/include, please install the package file located at:

/Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

[ilovezfs]

😭

[mistydemeo]

@andrewpile Yeah, I'm planning on replacing this PR with one that diretly addresses that... thanks!

[yurikoles]

rdar://40833474

How to open that link $_%?

[mistydemeo]

How to open that link $_%?

Radars aren't public; I included the radar number for reference, but it's not accessible.

[mistydemeo]

Closing in favour of #4334 and #4335, which are more thorough approaches to this situation.