Q:

Install Ruby with Mountain Lion, Xcode 4.5, and RVM? 

rvm install 1.9.3 --with-gcc=clang (as suggested by Can't install Ruby under Lion with RVM – GCC issues) doesn't work because neither Mac OS X 10.8.2 (Mountain Lion) nor Xcode 4.5 comes with clang. 
Is it possible to compile Ruby with llvm? 
According to Matthias Schmidt's "How to install Ruby 1.9.3 on Mac OS X with LLVM and rbenv", Ruby is now fully compatible with LLVM. 
But, I still get: error: C compiler cannot create executables
UPDATE:
Actually, I found clang here /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang (thanks to the Node.js Installation Wiki).  I just didn't have clang in my path. 
So, if I add clang to my path, is rvm install 1.9.3 --with-gcc=clang sufficient? 

A:

From RVM notes:
Homebrew:
If you are using Homebrew, you can install the apple-gcc42 and required libraries from homebrew/dupes:
  brew update
  brew tap homebrew/dupes
  brew install autoconf automake apple-gcc42
  rvm pkg install openssl

This can live side by side with an existing Xcode 4.2+ install or Command Line Tools for Xcode. 
osx-gcc-installer:
If you don't use Homebrew, you can download and install osx-gcc-installer:
https://github.com/kennethreitz/osx-gcc-installer. 
Hope it will help you. 

A:

no, ruby is not fully compatible with llvm, there is at least one issue with Fibers and other issues might occur if version of llvm changes. 
Ruby works best with gcc-4.2, rvm provides information how to get it (+other important stuff):
rvm requirements

