SERVER-9855 Remove obsolete darwin configuration options

- We want to use whatever compiler SCons tooling picks now that we support clang.
- /usr/64 isn't a thing on modern Darwin (at least since 10.6, which is our minimum)
- It probably never made sense to look in /usr/64/include for anything
- If you want to use MacPorts, use --extrapath=/opt/local, similar for Fink
SERVER-9855 Add support for specifying Mavericks as the OS X minimum version