Currently it is hard to avoid having LLVM link to the system install of ncurses, since it uses check_library_exists to find e.g. libtinfo and not find_library or find_package.
With this change the ncurses lib is found with find_library, which also considers CMAKE_PREFIX_PATH. This solves an issue for the spack package manager, where we want to use the zlib installed by spack, and spack provides the CMAKE_PREFIX_PATH for it.
This is a similar change as https://reviews.llvm.org/D79219, which just landed in master.