Treat headers found through cbdeps as system headers

Some compilers supports an option to treat some directories
as system headers and would _not_ emit warnings from them.
gcc/g++/clang/clang++ supports this option.

Change-Id: I5e4af6f20ae894dcb3b1056435da22089e15f571
Reviewed-on: http://review.couchbase.org/44092
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: buildbot <build@couchbase.com>