[builtins][CMake] Replace custom target for lse_builtin symlinks

[petrhosek]

Using custom target with BYPRODUCTS results in symlinks being recreated on every build (the build is always dirty). We could instead use custom commands to generate the symlinks and by adding the output to the list of sources, build system will ensure those are created as needed.

[mstorsjo]

Thanks for this, I've also been annoyed by these being remade all the time. I don't really know the details about the cmake constructs at work here though, so I'll defer the review to @smeenai.

[smeenai]

Never mind, I looked at the whole file and understand this better now. We're still adding the generated files to the sources. I thought there might be issues doing that with non-Ninja generators, but https://cmake.org/cmake/help/latest/command/add_custom_command.html#examples-generating-files seems to explicitly bless it, so we should be good.

The only thing that stood out to me on that same page was:

Each source file may have at most one command specifying it as its main dependency.

Which we're violating. Dunno if it matters in practice, but we could just use DEPENDS instead?

[petrhosek]

The only thing that stood out to me on that same page was:

Each source file may have at most one command specifying it as its main dependency.

Which we're violating. Dunno if it matters in practice, but we could just use DEPENDS instead?

Thanks for catching that, I missed this part. I changed MAIN_DEPENDENCY to DEPENDS.