[CMake] Formalize whether libdispatch is used/allowed

[davezarzycki]

Accidentally requiring libdispatch is a repeated cmake regression. Let's avoid this entirely by formalizing whether libdispatch is enabled or not rather than infer it.

[davezarzycki]

@swift-ci please test

[davezarzycki]

@buttaface – I think I've addressed your concerns. Feedback would be appreciated.

@swift-ci please test

[finagolfin]

Nice work simplifying the logic.

[compnerd]

This effectively disables the use of SWIFT_BUILD_SYNTAXPARSERLIB and SWIFT_BUILD_SOURCEKIT if SWIFT_ENABLE_DISPATCH is false. I think that if you want to expose the explicit control over dispatch, please use cmake_dependent_option to ensure that the user cannot set SWIFT_BUILD_SYNTAXPARSERLIB or SWIFT_BUILD_SOURCEKIT while SWIFT_ENABLE_DISPATCH is a false value.

[davezarzycki]

@compnerd – Thanks for the feedback. The patch now uses cmake_dependent_option.

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 81b6acaabf1e9f320467c4fc7883716364776ab2

[davezarzycki]

Hey @compnerd – It looks like that CMake feature is too new. From one of the Linux builders:

12:34:19 CMake Error at CMakeLists.txt:413 (cmake_dependent_option):
12:34:19   Unknown CMake command "cmake_dependent_option".

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 81b6acaabf1e9f320467c4fc7883716364776ab2

[compnerd]

@davezarzycki I suspect you missed the include(CMakeDependentOption) which actually is required. The feature has been there for a very long time (3.13 at the very least).

[davezarzycki]

@compnerd – Ah that must be it. Fixed.

@swift-ci please test

[compnerd]

@swift-ci please test Windows platform

[compnerd]

I think that this shouldn't change the default behaviour and still allow control over dispatch and validate inputs, so seems reasonable. Please ensure that it passes the Windows CI as well (though I don't see why it wouldn't).