[SR-2653] Allow CMake SWIFT_BUILD_SOURCEKIT to be enabled/disabled via build-script

[modocache]

Previous ID	SR-2653
Radar	None
Original Reporter	@modocache
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Project Infrastructure
Labels	Improvement, StarterBug
Assignee	None
Priority	Medium

md5: 9e97a17cc680616ce092d753d43de0a3

Issue Description:

I'm currently attempting to cross-compile the stdlib for Android using a macOS host. One snag is that apple/swift/CMakeLists.txt adds a SourceKit target if the host machine is Darwin, and does not add it if the host is Linux. I should be able to turn SourceKit builds on or off via the build-script.

Following the project's convention, this should probably be an option named --skip-build-sourcekit. On Linux, utils/build-script should pass this option by default. On Darwin, it should not.

[modocache]

Scratching my own itch here: #4806

[modocache]

Apparently my approach isn't ideal here. Others should feel free to try to do it right, I might not be able to update my pull request for another few days.