To build on Mac OSX, you will need to download and compile your own boost, leveldb using gcc 4.7 before building libbitcoin. Then set the following environment variables before running the build process.
CC=/usr/local/bin/gcc-4.7
CXX=/usr/local/bin/g++-4.7
For Mac OSX Mountain Lion, you need to follow these brew commands:
$ brew install boost openssl leveldb curl