Trouble installing bcrypt-ruby 3.0.1 on Mointain Lion where it can't find your gcc-4.2?
The path to gcc-4.2 is hardcoded in the Makefile.
Fix: Symlink /usr/local/bin/gcc-4.2 to /usr/bin/gcc-4.2
$ sudo ln -s /usr/local/bin/gcc-4.2 /usr/bin/gcc-4.2
I think this only works if you have gcc installled via homebrew. If gcc is installed as part of command line xcode tools than you need different symlink:
sudo ln -s /usr/bin/gcc /usr/bin/gcc-4.2