This adds a third_party/boost folder with a CMakeLists.txt. If this
folder is added via add_subdirectory, it downloads boost to the
binary directory and exposes the Boost::boost target.

Bug: b/145758253
Change-Id: I7d4e383630cb84da928cf0622a641e9e0efc2ad8
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43191Reviewed-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>