There is no "release" of gcc-4.8 yet, and my experience (not Open MPI related) with recent snapshots has encountered many bugs still to be fixed before any release. Even if you could build Open MPI with gcc-4.8, I would not (at this time) trust it with any "production" jobs.
Looking at the config.log you attached, it looks like your installation of gcc is unable to run the "as" assembler, which probably means you don't have Apple Xcode installed (which is the preferred source of the assembler and linker on Mac OSX). So, I doubt that you can even build a "Hello, World!" C program with your gcc installation - have you tried?