Make mac script run idempotently

Use preferred POSIX `command -v` test for command existence instead of
previous `which` technique. `which` exits with status `1` if nothing is
found. That would cause the script to abort because we use `set -e`.

http://stackoverflow.com/questions/592620/how-to-check-if-a-program-exists-from-a-bash-script

Remove GCC dependency in mac script

GCC is no longer necessary to compile Ruby. It was fixed in Ruby about
two years ago. LLVM has been supported since 1.9.3-p125.

https://bugs.ruby-lang.org/issues/5076
http://stackoverflow.com/questions/8032824/cant-install-ruby-under-lion-with-rvm-gcc-issues