change build_detect_platform to check for OS X Mavericks

On Mavericks clang is the default compiler, so modify build_detect_platform
to detect that OS X version and set the necessary compiler and linker
switches.

Successfully tested on Mavericks, Mountain Lion, and Lion.

I strongly suggest rebasing this to master rather than merging, as that way the corresponding changes in the eleveldb c_src/build_deps.sh script will continue to pick up the right leveldb commit sha created by this commit.