Created by: sheldonneuberger
Fixes https://github.com/facebook/buck/issues/1906
Adds a system_include parameter to cxx_library that defaults to false. When set to true, that library's exported headers will be included with "-isystem" instead of "-I" by consumers. The practical effect of this is to ignore warnings from those header files.
prebuilt_cxx_library includes its headers with -isystem, but it's common to copy the entire source of a project into the repo and build it with cxx_library, in which case system_include=True comes in handy when you want to avoid making edits to the source.