libLLVM.dylib missing from usr-staging

[vtjnash]

it looks like libLLVM isn't getting installed correctly anymore (to usr-staging) after some recent change and is now incorrectly getting installed straight to the usr directory

[vtjnash]

@tkelman caused by #16761 (it fails to respect DESTDIR). also, any idea if the build is working with cmake (since the fix in that PR wouldn't be applied then)?

[tkelman]

what build? cmake shared library on windows? no, not without a hacky patch, needs proper fixing upstream.

I'm still not convinved the whole usr-staging business fixes any real issues to be worth the confusion and how error-prone it is.

[ihnorton]

@tkelman re LLVM as a shared library on Windows via CMake: CMake's new(ish) WINDOWS_EXPORT_ALL_SYMBOLS might help.

[tkelman]

924049a was a hacky attempt but it needs work