Specify missing dependencies on swift_static .swiftmodule files

[kubamracek]

Apparently, the .swiftmodule files in "lib/swift_static" (as opposed to "lib/swift") do not get incrementally rebuilt on changes to .swift source code in the stdlib. I think I understand CMake less and less, if someone could explain to me why this is needed I would be forever grateful :)

[compnerd]

This is desired behaviour! There is no reason for regenerating the swiftmodule, the custom command has no dependency on the sources, so if the sources changes, there is no change to the swift module as far as you have informed CMake. This is the same as having a Makefile rule that generates a file but does not depend on its inputs - it would never change. This is why explicitly listing source files in add_library and add_executable is important - it ensures that any change will re-trigger the target to be built.

I would say that it's the homegrown build system written in CMake that you don't understand rather than CMake, CMake is not really meant to be used this way.

[kubamracek]

I guess the part that surprises me is this (and please correct me if my understanding is wrong): If the specified dependency is on a file, it behaves differently from a dependency on another target. A change of the file triggers a rebuild, but a "rebuild" of target dependency does not trigger a rebuild. In that sense, the dependency-rebuilding behavior is not transitive (as I had naively expected).

[kubamracek]

@swift-ci please test