-std=c99 is not accepted by clang c++

Change-Id: Iac3fa29b709892bdffa2295450d1b026a7a4b7ea
Reviewed-on: http://review.couchbase.org/26902
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>