After installing Developer Command Line Tools for OS X, you should have basic tools like clang and git installed. After that, you can grab other required packages via Homebrew. So as in instructions on the project page:
ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
And then:
brew install cmake sdl sdl_image sdl_ttf boost glew physfs flac libsndfile libvorbis vorbis-tools gettext po4a
Gettext is installed in separate directory without adding the files to system path, so in order to get it working normally, you should call also:
brew link gettext --force
When installing po4a, you might get into trouble over dependency of SGML module for perl. The best way is to install it manually, which requires some changes in SGML's package Makefile. Appropriate instructions can be found here.
If you've installed everything correctly, the simple way of compiling Colobot with CMake should work:
git clone --recursive https://github.com/colobot/colobot.git
mkdir colobot/build
cd colobot/build
cmake ../
make