Get an error with unknown argument 'multiply_definedsuppress'. Xcode has been recently changed to fail when given unknown arguments. For more information please refer to the following

http://bruteforce.gr/bypassing-clang-error-unknown-argument.html

Complied successfully are using the following workaround

`ARCHFLAGS=-Wno-error=unused-command-line-argument-hard-error-in-future gem install bson_ext`

compiling bson_buffer.c
compiling cbson.c
compiling encoding_helpers.c
linking shared-object bson_ext/cbson.bundle
clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
make: *** [cbson.bundle] Error 1

make failed, exit code 2